Nick Rutledge

Soon after it touched down Monday, a tornado plowed through Newcastle, Okla., as it made its way along a 17-mile path.

Early Monday afternoon at the Shear Perfection hair salon, a stylist named Lisa Lentz decided to outrace it. Her one o’clock, a cut-and-color, was done, but two other clients had just canceled, and the ominous tone of Gary England, the meteorological oracle on News 9, commanded attention.

Ms. Lentz, 47, left the other stylists, who would soon be praying in the lemon-scented bathroom, plastic baskets over their heads like combat helmets. She hustled to her old minivan, redolent of pampered dogs and countless family journeys, and looked to the western sky: like smoldering charcoal, and all too familiar.

Not 10 miles to the southwest, in a two-story brick house in the small city of Newcastle, Shelly Codner, 50, was making the same reacquaintance, only from a closer angle. Two televisions were shouting a duet of warning, and an alert on her iPhone was saying the thing was five miles from the nearby Newcastle Casino.

Time to grab the emergency bucket, packed with flashlights and glow sticks, bottled water and food for dogs and baby. Time to head to the steel-encased storm shelter built into the garage floor last summer, with husband, teenage daughter, 11-month-old granddaughter, and all five dogs. Oh, and a couple of Fisher-Price distractions.

Here we go. Again.

In this breeding ground of Oklahoma tornadoes, people prepare for the season with the care that the defensive coordinator for their Sooners prepares for the inevitable autumn. They develop family plans, hang on the words of meteorologists, and, in places like Moore, become accustomed to the Saturday noontime testing of emergency sirens. At the same time there exists disbelief that the devastation visited upon neighbors could ever happen to them or, that is, could ever happen to them again.

Amid all the siren tests and awareness and false alarms, the warning can still be a half-hour, maybe a little more, maybe a little less. This means you must stop what you are doing, shake off the disbelief, track down loved ones and find shelter, all in the time it takes to watch a few rounds of “Jeopardy!”

A half-hour or so is not much, but the alert system saves lives. This monstrous tornado would kill 24 people, but in an earlier, low-tech time, the number of dead would certainly have been much higher.

As Ms. Lentz pulled out of the strip mall’s lot and turned right onto South Western Avenue, she saw to her right a pulled ashen curtain in the sky — the creation of the collision between cool, dry air down from Canada and warm, moist air up from the Gulf of Mexico, helped along by the strong jet-stream winds over the Oklahoma plains. This natural alchemy had created a whirling vortex, awesome, destructive, seemingly alive.

Despite the caprice of its path, she still found herself thinking as she drove on, “It’s not going to hit you.”

But tornadoes are not benign optical illusions. Thirteen miles to the southeast, on the University of Oklahoma campus in Norman, a shift of a half-dozen forecasters at the National Weather Service’s storm-prediction center was continuing the never-ending monitoring of heavy weather across the continental United States. The focus at the moment was their backyard.

They had spotted the trouble a week in advance. Synthesizing information from satellite and radar, computer models and ground-level observations, the center warned on Wednesday, May 15, of severe weather headed for the region, pointing to Monday, May 20, as having perhaps the “highest tornado potential.”

Thursday, Friday and Saturday provided no reason to alter the prediction. On Sunday, the center tightened the potential target area, warning of a “threat for very large hail and tornadoes” from central Oklahoma into west central Missouri.

By early Monday morning, the “warning coordination meteorologist” for the local forecast office of the National Weather Service, Rick Smith, could not shake a bad feeling about the day. After checking the latest worrisome data, he sent an e-mail at 8:06 to 380 government, hospital and emergency-management officials in the region, signaling the strong likelihood of tornadoes in the early afternoon, right around dismissal time at the schools.

Gordon M. Grant for The New York Times

The restoration and retrofit of the 1881 brick Bulova factory in Sag Harbor into 64 residences is expected to be completed next winter. Construction began in the fall of 2011.

The project, built by Cape Advisors, a developer based in Manhattan, was designed by the architectural firm Beyer Blinder Belle. With its high-beamed ceilings and exposed brick walls, it is something of an anomaly in this Hamptons community dominated by single-family homes.

The factory sat vacant as an eyesore near the heart of Sag Harbor for years. Construction on the condos, which will include lofts, town houses and bungalows, began in the fall 2011 and is expected to be completed next winter. The first model apartment, a $3.39 million two-bedroom penthouse, opens this weekend. James Lansill, a senior managing director of the Corcoran Sunshine Marketing Group, said that 880 potential buyers already fill a five-year-old waiting list.

“It is unprecedented,” Mr. Lansill said, referring to the historic retrofit and the advent of a deluxe condominium with a doorman, on-site superintendent and resort-style amenities. “There is barely such a thing as a condo in the Hamptons.”

Among those on the waiting list are owners of multimillion-dollar Hamptons mansions, including empty-nesters looking for something easier to take care of, without the need for a staff, pool guys and gardeners. Longtime seasonal renters who didn’t previously buy because of maintenance responsibilities, particularly in the off-season, have also signed up. The distinctive units will carry hefty price tags. Factory lofts will range from $1.05 million to $3.22 million, and penthouses from $2.59 million to $10.2 million.

In addition, 17 bungalows and town houses designed to complement the factory building will be placed atop, though on the perimeter, of a 130-car underground garage that is being built under what was a vacant parking lot. Amenities, including a swimming pool, a fitness center and a gardens, are also being created over the garage. John H. Beyer, a partner in Beyer Blinder Belle, which also handled the restoration of historic Grand Central Terminal, described the old Bulova factory as an “industrial version of a Victorian mill.” The five-story solid brick structure has “modest but very effective detailing.”

After the whaling industry here declined in the mid-19th century, the Watchcase factory, with wings added in five stages over its 100-year history, played an important role in the village’s development, including the sponsorship of blue-collar housing. The largest of Sag Harbor’s factories, the Watchcase employed generations of local workers beginning in 1881, when Joseph Fahys, a French immigrant who had married a local woman, relocated his watchmaking factory from Carlstadt, N.J., to Sag Harbor. It replaced a cotton mill built by former whalers and destroyed by fire.

Bulova subsequently bought the factory in 1936. After the factory shut down in 1981, the building stood empty, its gutters dangling, windows broken and crumbling bricks defaced with graffiti. Cleanup of contaminants from a century’s worth of heavy cleaning solvents poured into the building’s drainage system began in the 1990s. Asbestos removal was completed before the retrofit began.

Despite its long industrial history, Mr. Beyer said he was “stunned by the appropriateness of introducing residential uses into this factory,” with its 24-foot-wide wings and “endless repetitive, beautiful, spacious windows.” The building has 999 arched windows, each three feet wide and seven and a half feet high. Many of the apartments will have more than 20 windows. The way the building was sited, with wings stretching along Washington, Division and Church Streets, lent itself to “creating courtyards and spacious gardens.” The old boiler room was removed to create the main lobby, with courtyard space in front of it, between two wings.

Steven Gambrel, an interior designer working with the developer, kept the structure’s “muscular architecture” in mind. “It is not a beach house and it is not a maritime house,” Mr. Gambrel said. A huge two-sided fireplace is being carved into the original factory smokestack in a lounge area off the main lobby.

Some of the turrets, chimneys and an old water tower that fell into disrepair during the three decades the building stood vacant are being replaced, according to Craig D. Wood, a chief executive of Cape Advisors. Each of its 999 brick window openings were repointed, with 20,000 original bricks scraped clean and reused. A bracketed three-foot cornice replicating the factory’s original, which was removed about 50 years ago, is also being installed. Inside, the original southern yellow pine timber beams were blasted with 80,000 pounds of ground walnut shells to restore their patina in an eco-friendly fashion.

The state charges a recording tax on new mortgage debt. The rate varies by county, with the minimum being 1.05 percent of the loan amount. The rate is highest in New York City, where borrowers pay 1.8 percent of the loan amount for mortgages under $500,000, and 1.925 percent above that amount.

A city resident borrowing $600,000, for example, would be charged around $11,500 for the recording tax.

But fortunately, homeowners aren’t required to pay the tax again when they refinance. “The borrower already paid the tax on the existing mortgage and is entitled by statute to an exemption from payment of the tax with respect to an existing principal balance a second time,” said Guy Arad, a lawyer with Adam Leitman Bailey in Manhattan.

Still, borrowers who choose to switch lenders when refinancing sometimes get stuck paying it anyway.

Here’s why: in order to skip the tax when switching lenders, borrowers must arrange for their existing lender to assign, or transfer, the mortgage to the new lender. The new lender then recasts the old mortgage to meet the new terms. But lenders don’t always agree to do what’s known as an assignment — a 1989 amendment to state law gives them discretion to reject such requests, according to Douglas Wasser, a Manhattan real estate lawyer.

“The resulting cost to borrowers can be thousands of dollars in taxes which could otherwise be easily avoided,” Mr. Wasser said. “I’ve personally seen borrower frustration on this issue many times.”

Rolan Shnayder, a partner and the director of new-development lending for H.O.M.E. Mortgage Bankers in Manhattan, says this problem arises “all the time.” In his opinion, some banks refuse to do transfers to encourage customers to refinance with them. “Even if their bank is offering the best interest rate, what if the borrower just doesn’t want to deal with their bank anymore, or wants to transfer to a different lender because they have other business there?” Mr. Shnayder said. “There are a lot of customers who wish they’d known their lender wouldn’t do an assignment before they signed their mortgage.”

The assignment process (a Consolidation, Extension and Modification Agreement) requires more time and paperwork than the usual practice of just paying off the old mortgage. And both the old and new lenders must be represented at the closing table, Mr. Wasser said.

Borrowers will likely have to pay extra legal fees, along with the old lender’s assignment fee. Those fees should be weighed against the tax savings.

“I tell clients, if the mortgage tax savings are less than a couple of thousand dollars, the time, effort, aggravation and cost may not be worth the result,” Mr. Wasser said.

Also, borrowers should be aware that if their new loan is larger than the outstanding debt on the previous loan, they will be taxed on the difference. For example, if the unpaid balance on an old loan is $300,000, and the new loan is for $500,000, the borrower will be taxed on $200,000 in new mortgage debt.

Mr. Wasser recommends that borrowers always ask what a lender’s policy is on transfers before signing on. “An educated borrower,” he said, “might know that uncooperative lenders should be avoided.”

Q Owners in our eight-unit condominium building have had problems trying to sell their apartments because banks will not lend to potential buyers when owner occupancy drops below 50 percent. Right now four of our units are owner-occupied and four are rented out. Should we amend our bylaws to make sure that owner-occupancy is always above 50 percent? Is this legal?

A It is legal, and amending the bylaws probably makes sense, said Matthew J. Zangwill, a real estate lawyer in Uniondale.

Mr. Zangwill said that the writer was correct in stating that lenders are generally hesitant to provide financing in buildings with a high percentage of nonowner-occupied apartments. “And the problem can be exacerbated when a condominium has a small number of units like this one,” he said.

Mr. Zangwill said that it is possible for unit owners to make the building more attractive to lenders by amending the condominium bylaws to limit the percentage of apartments that can be rented out, or even eliminating the right to do so entirely. But he noted that state law requires an affirmative vote of at least two-thirds of unit owners to amend the condominium’s governing documents. Because four apartments are already rented out, it may be difficult for the owner-occupants to get the votes necessary to make the change.

Moving Out Without an Elevator

Q My landlord is renovating our elevators from the beginning of July to the end of August. My lease expires on July 31, and building management says that if I will not be renewing my lease, I will have to move either before July 1 or after Aug. 24 if I need to use the elevator. Are they allowed to do that? They say they want me to remain as a tenant. Does this give me any leverage in negotiating a renewal lease?

A Dov Treiman, a Manhattan real estate lawyer, said that the reader seems to be making an assumption that an elevator is necessary for the move.

“While on the practical level, this is often the case, the law does not look at it that way unless the elevator is genuinely the only way a tenant can move,” Mr. Treiman said. “In most cases, the staircase is an unpleasant but usable alternative.”

He said that the inability to use the elevator might give the tenant a reason for remaining in occupancy after the lease expires and withholding the rent until the elevator is back in service. He added, however, that it is unlikely that the situation would give the tenant any “leverage” in negotiating a renewal lease.

Rejected Because of Low Earnings

Q I am having trouble renting in New York City because I am a senior who works part time earning about $13,000 a year. Though I have assets of about $400,000, two landlords have rejected me because my income is too low. Is there anything I can do?

A Federal, state and local laws make it illegal for a landlord to deny housing applications based on an individual’s age, but owners are within their rights to reject a prospective tenant who is unable to demonstrate a financial ability to make the required rent payments, said Randolph M. McLaughlin, a Manhattan civil rights lawyer and a professor at Pace Law School.

While there is no set formula for determining that, he said, many landlords require that a tenant’s annual incomes equal or exceed a certain “multiplier” which can be as high as 80 times the monthly rent. So, for example, if the monthly rent were $1,000, and the landlord used a multiplier of 80 times the rent, the letter-writer would be expected to show an annual income of at least $80,000 a year.

Submit questions to Q & A by e-mail to realestateqa@nytimes.com

But before the 1850s, Fifth Avenue was barely a notch above its easterly colleague, and today the observant may scrape up the few traces of gentility amid the falafel and the fraternity crowds.

Second Avenue opened after the adoption of the grid plan in 1811, and wealthy families put up comfortable brick Greek Revival houses, like the Isaac Hopper house at 110 Second Avenue, nearly intact from the 1830s. That the diagonal of Stuyvesant Street ran right into Second at 10th Street didn’t hurt — the grand 1804 Stuyvesant-Fish house is just west of Second.

Indeed, the Stuyvesant family courses all through this section: Peter Gerard Stuyvesant’s 1845 house stood at the northwest corner of Second and 11th Street. It soon went to Lewis Morris Rutherfurd (sometimes spelled Rutherford), another member of the clan, and he put an observatory in the backyard, from which he observed eclipses and other heavenly phenomena.

Another town house from the 1840s is 149 Second Avenue; it still has its stoop and is comparatively little altered, although there’s a big skylight on the roof that must give some apartment plenty of sun. The 1870 census records the occupants as Edward Jaffray, a socially prominent dry goods importer, his family of five and nine servants.

Two years later, Jaffray advertised his “extra-sized house” for sale. Thirty-three feet wide, it had three bathrooms, six water closets and the property included a stable on Ninth Street.

A subsequent owner was Alexander Major, a founder of the New York Yacht Club, who lived there with his family of five and seven servants. According to Vanessa Cameron, the archivist of the club, George Steers, who designed the cup-winning America, also did Major’s yacht.

But the precincts of Second Avenue were tarnishing, and as early as 1853, Gerard Stuyvesant and others had to protest the extension of a surface railway along Second, which a later map shows.

And in the financial panic of 1873, a plumbing contractor who was a member of the Tweed ring, John H. Keyser, fed 1,000 people a day out of his house on Second Avenue. Public-spirited, yes, but perhaps he was also out to untarnish his image.

Simultaneously, German immigrants took over the old houses, converting them to apartments and later, tearing some down for tenements and night spots. No. 138 Second Avenue, a house which boasts a very nice surviving brownstone doorway with a Gibbs surround, had by 1885 become the Association for Befriending Children and Young Girls.

In 1912, The New York Times noted that a longtime Second Avenue resident, Alice Keteltas, “drove away one afternoon in her coach from the aristocratic mansion on the northwest corner of St. Marks Place, never to return there.” According to the newspaper, “the neighborhood, teeming with a mixed Jewish, Italian, and Hungarian population, has never seemed to bother the old lady.” But in the end she was persuaded by her friends to decamp to East 79th Street and presumably, continued to vacation at her house in Newport, R.I.

Maria Major, the widow of Alexander, died in 1917 in her house, 149 Second Avenue. A year later, The Eagle reported that it had been “the only private residence remaining in this once fashionable avenue.” In the 1930s the house was occupied by the Yiddish Workers Culture Chorus.

There is still some of the old egg-cream-ethnic left on Second Avenue, but now the chief cultural group is 20-something singles, who spill onto the sidewalks like a giant fraternity party, more ebullient than disorderly, even with plenty of beer.

Any of them of a mind to put down the iPhone and search out traces of the old, old Second Avenue — that is, the mansion period — will be hard put to find it. Besides the Gibbs surround at No. 138, a rare find, there’s a row of brick houses at the southwest corner of Third Street sitting in prim rectitude among the tumult. Some of the houses are badly mangled by two-story storefronts, but a few conceivably contain old pier mirrors or what’s left of elegant double parlors with twin columns. The Hopper house at No. 110 is in this group.

There is also New York Marble Cemetery, hidden away on the west side, from Second to Third Streets. Its interment roster is redolent of a New York so long gone it is a foreign country: Abeel, Anthon, Devoe, Hosack, Luqueer, Vandervoort.

Despite a ragged storefront, the old Major house at No. 149 prevails, the soft, worn red brick of the upper floors like some beautiful stones smoothed for decades in a mountain stream. The Greek Revival ironwork on the stoop is damaged, but if you mount the stairs you will be rewarded by a real madeleine of old Second Avenue.

There, through a fuzzy pane of scratched glass, the idler can see a broad set of wooden stairs trailing away up to the second floor, which has a wide midlevel landing. Around the ceiling of the vestibule runs a confectionery of cove molding, delicate little free-standing plaster flowers, impossibly fragile, but impossibly intact, at least in most places.

According to a tenant, the interiors have been much altered, but these fragments were here on Maria Major’s final trip from the Second Avenue of a much, much earlier time.

Hans Deryk/Reuters

Casa Casuarina in Miami, the former Gianni Versace mansion, was listed by telecom mogul Peter T. Loftin for $125 million.

Billionaires found the green-carpet views of Central Park — meticulously captured by a remote-control helicopter — to be irresistible. Gary Barnett, the president of the Extell Development Company, announced sales of full-floor apartments for more than $50 million apiece in the unfinished building, later confirming that the duplex penthouse and an even larger duplex 14 floors below had each gone into contract for more than $90 million.

Success begat boldness. And boldness begat listings like the penthouse at CitySpire, which at $100 million is now perceived as the highest F.S.B.O. listing in New York history, although the owner, Steven Klar, has listed the property with his own brokerage. The sales contract that Mr. Klar, who is also a developer, signed with Douglas Elliman and its brokers Raphael De Niro and Victoria Logvinsky, has expired, and now Mr. Klar is trying to sell the 8,000-square-foot octagonal residence himself. (An Elliman spokeswoman had no comment this week on whether any serious offers had ever been received.)

New York was not alone. In Miami, owners and their agents took their cues from the frenzy seizing Manhattan. The developer Bruce Eichner listed his penthouse at the Continuum for $39 million. The telecom mogul Peter T. Loftin listed Casa Casuarina, the former Gianni Versace mansion, for $125 million, dropping the price five months later to $100 million. The buyers of Mr. Loftin’s 23,000-square-foot spread on Ocean Drive will have to tangle with a pile of litigation; the latest suit, from Barton G. Weiss, the restaurateur who has been operating the property as a hotel, revealed that the 54-foot mosaic-tiled swimming pool lined in 24-carat gold was never approved for use by hotel guests.

None of those properties have sold yet. And others with eye-popping price tags in Manhattan, including co-ops at the Sherry-Netherland and the Pierre, as well as Leroy Schecter’s combo unit at 15 Central Park West (recently reduced to $85 million from $95 million), continue to collect dust waiting for a trophy collector.

If there is a defining feature of the last year, it is that the mania over listing prices for trophy properties was built on a mirage of big sales creating new comps in the market. Consider that in Manhattan only one monster sale above $80 million — the $88 million sale of a penthouse at 15 Central Park West — has actually been recorded as a sale so far. The others are contracts that could be torn up if the prospective buyers somehow change their minds.

Which isn’t to say that extremely wealthy people aren’t paying extraordinary prices for amazing apartments in New York and Miami. It’s just that the mega-rich in both cities are gravitating not to resales, but to spectacular new penthouses designed with them in mind. Properties like the duplex penthouses at Ian Schrager’s Residences at the Miami Beach Edition, which sold as a package for $34 million (or $3,800 a square foot, a new Miami Beach record), or the penthouse at 432 Park Avenue, which the developer Harry Macklowe said is under contract for $95 million, continue to prove the point.

“The only properties that are getting these numbers are new development projects, and those can’t be confirmed until the year after when they close,” said Jonathan J. Miller, the president of Miller Samuel, a property appraiser. “So in many ways this is a phenomenon that is hard to document or prove. However, it has launched the next wave of development.”

And that new wave of development, in New York, at least, is focused almost entirely on the top 10 percent of the market, Mr. Miller said, as developers feel pressured to build luxury units to justify soaring land prices.

Still, it may well be that the trophy property craze spawned a wave of transactions for more modest — believe it or not — $20 million and $30 million apartments.

But the nature of the adaptation that researchers in North Carolina described on Thursday in the journal Science is impressive even for such an ancient, ineradicable lineage, experts say. Some populations of cockroaches evolved a simple, highly effective defense against sweet-tasting poison baits: They switched their internal chemistry around so that glucose, a form of sugar that is a sweet come-hither to countless forms of life, tastes bitter.

The way the roach's senses changed, experts say, is an elegant example of quick evolutionary change in behavior, and offers the multibillion-dollar pest control industry valuable insights into enemy secrets, perhaps even revealing some clues for the fight against malaria-carrying mosquitoes, which are far more dangerous to human health than roaches.

“This is a fantastic discovery,” said Walter S. Leal, the head of the entomology department in the College of Agricultural and Environmental Sciences at the University of California, Davis. (Dr. Leal was not part of the research.)

“Sometimes,” he said, “the science is beautiful but you don’t know whether there is going to be an application five years from now, 10 years from now or 100 years.” But in this case, he said, the impact was both fundamental and practical.

Ayako Wada-Katsumata, Jules Silverman and Coby Schal, all at North Carolina State University, who wrote the report in Science, set out to explain a well-known phenomenon: Some populations of German cockroaches (the ones that apartment dwellers see scurrying around in the kitchen at night) avoid poison bait that is laced with glucose, which is supposed to attract them.

This behavior, discovered by Dr. Silverman, "first appeared in the early '90s," said Jim Fredericks, chief entomologist at the National Pest Management Association, shortly after exterminators — who now prefer to be called pest management professionals — started using poison baits instead of spraying as the main method of battling roaches. To get around the problem, the industry developed new baits, but the change in roach behavior was a puzzle.

Grzegorz Buczkowski, an entomologist at Purdue University who was not involved in the research, said the industry was always developing new poisons, because roaches and other pests become resistant to their effects, just as bacteria become resistant to antibiotics.

“We lose baits all the time,” he said.

But in this case, the problem was not a poison that had become ineffective. The cockroaches just seemed to avoid any bait that had glucose.

Dr. Silverman showed that this behavior was inherited, not something an individual roach learned during its brief life. And a few years ago the North Carolina researchers decided to investigate what caused the change.

Instead of taste buds, roaches have taste hairs on many parts of their bodies. The three North Carolina researchers concentrated on those around the mouth area and on two types of nerve cells that sense tastes and respond by firing electrical signals to the brain. One responds only to sugars and other sweet substances; the other responds only to bitter substances. Whenever a molecule of something sweet attaches to a sweet detector, it fires electrical impulses and the roach brain senses sweetness, which makes it want to eat whatever it is tasting. Whenever a molecule of something bitter attaches to the bitter detector, that cell fires and the brain senses bitterness, which makes the roach want to avoid that substance.

But somehow the roaches had changed so that the glucose made the bitter detector fire.

“Basically,” said Dr. Buczkowski, “when cockroaches taste glucose, they’re repelled by it because it tastes bitter to them.”

Dr. Schal said the next step was to figure out the details of the genetic mutation that occurred. Perhaps a mutation changed the molecules that detect bitter substances so that they would be sensitive to glucose too. Or a different sort of mutation could have caused the dedicated bitter neurons to have lots of standard glucose detectors, which did not exist on those neurons before — a shift that also would have made the insects register sweet glucose as bitter.

The research may be relevant far beyond roach control, perhaps helping to explain the behavior of mosquitoes that spread malaria, Dr. Schal said.

“The mosquito changed its behavior,” he said, “and no longer rests on walls that are treated with insecticide. Instead it tends to rest on the ceiling, or it tends to rest on the outside walls that are not treated with insecticide.

“We still don’t understand the cellular, the neural mechanism responsible for this change in behavior of the mosquito,” he said, so the approach that yielded results with the cockroach could offer useful insights.

Ángel Franco/The New York Times

The chlorine atoms that help keep pools like the one at Barretto Point Park in the Bronx, pictured here in 2012, from becoming a microbe-choked petri dish were forged billions of years ago in ancient stars.

I speak, of course, of chlorine.

Over the next few months, chlorine will ensure that countless swimming pools don’t turn into microbe-choked petri dishes. That’s only one of many uses we’ve found for the element. We sprinkle it on our food as table salt — a k a sodium chloride. We pump water through pipes made of polyvinyl chloride. Perchlorate, a combination of chlorine and oxygen atoms, fuels rockets and ignites fireworks.

But in other incarnations, chlorine is a bane of our existence. In World War I, the German army unleashed clouds of chlorine gas and killed or injured thousands of enemy -soldiers. The Hudson River is burdened with cancer-causing dioxins, chlorine-bearing compounds dumped from factories along its banks.

Still, chlorine’s threats today are nothing compared with its menace on the early Earth.

According to a study recently published in the journal Earth and Planetary Science Letters, the entire planet was poisoned with the stuff. Fortunately, the planet got rid of most of its chlorine. If it hadn’t, we might not be here today.

“It would be tough to have any complex life,” said Zachary D. Sharp of the University of New Mexico, an author of the study.

Researchers like Dr. Sharp track the history of elements like chlorine to learn about how the solar system formed and how Earth became a habitable planet. The chlorine atoms floating in swimming pools today were forged billions of years ago in ancient stars. They drifted into a dusty cloud that would eventually become our solar system.

As Earth formed some 4.6 billion years ago, it swept up some of that chlorine and incorporated it into its rocks. To trace the history of chlorine on Earth, Dr. Sharp and David S. Draper of NASA Johnson Space Center recently tallied up how much of the stuff our planet contains.

The salty ocean contains a lot — about 20 million billion tons, in fact. More chlorine can be found in places like Death Valley, Calif., which formed from ancient evaporated seas.

It’s harder to know how much chlorine is hidden deeper underground. Some clues come from diamonds, which formed hundreds of miles underground before being pushed up to the surface. They also contain traces of chlorine.

But deeper down, things get more mysterious. “We have no samples from the core,” said Dr. Sharp.

We do know, however, that the Earth’s core formed when molten iron sank to the center of the planet. Lighter rock, like basalt, floated up to higher levels. So Dr. Sharp and Dr. Draper ran an experiment to see how chlorine would behave in such an environment.

They combined iron, basalt and chlorine and burned them under intense pressure. They found that all of the chlorine combined with the basalt, and none with the iron. Those results, Dr. Sharp and Dr. Draper concluded, mean that Earth’s iron core is chlorine-free.

All told, Dr. Sharp and Dr. Draper estimate the concentration of chlorine on Earth is 17 parts per million. And that’s a very puzzling number. Scientists have found that the meteorites that formed at the birth of the solar system have vastly higher levels of chlorine. Based on those studies, Dr. Sharp and Dr. Draper estimate that the early Earth had 10 times the amount of chlorine than it has today.

“A huge fraction of chlorine that should be on the Earth is gone,” said Dr. Sharp.

Dr. Sharp believes that most of Earth’s chlorine was wiped out by the huge meteorites that slammed into the planet during its first few hundred million years. Those impacts vaporized some of the oceans, sending chlorine out into space.

If those impacts hadn’t gotten rid of all that chlorine, Earth would be radically different. The oceans would have been loaded with as much salt as the Dead Sea. Such salty water wouldn’t be able to dissolve oxygen. Much less vapor would rise from it, leading to much less rain. The continents would have become parched, preventing nutrients like phosphorus from flowing from the land to the sea. “It would be a very nasty place,” said Dr. Sharp.

So nasty, in fact, that animals and other complex life-forms might not have been able to survive. A chlorine-laced swimming pool may be a pleasant place to spend a summer day, but a planet covered in a global Dead Sea would have been no vacation at all.

The Nuclear Regulation Authority said it agreed with the panel that the fault underneath the Tsuruga No. 2 reactor could set off an earthquake and cause an accident. Japanese regulations prohibit reactors from sitting above active faults. The No. 2 reactor at Tsuruga, one of two there, now faces an indefinite halt or likely decommissioning unless its operator provides new data on the fault.

It was the first time Japanese regulators had officially recognized an active fault underneath an existing reactor, virtually acknowledging that the risk at Tsuruga had been overlooked for decades by both the operator and regulators despite warnings by some experts. The watchdog is also investigating five other plants over suspected active faults.

The case is a crucial test for the agency to prove if it can resist industry pressure just as Japan’s pro-nuclear government moves to restart reactors suspended after the 2011 Fukushima disaster.

Even before the announcement on Wednesday, the chairman, Shunichi Tanaka, had hinted that his agency would not open a safety review for Tsuruga’s No. 2 reactor if its operator applied for an inspection ahead of a possible restart.

“Under the safety guidelines, we say a reactor should not be built on an active fault — it’s self-explanatory,” Mr. Tanaka said at a news conference. “We stand by our decision, no matter what kind of pressure we get from outside.”

Tsuruga’s operator, Japan Atomic Power Company, said it would continue its own study in hopes of overturning the assessment.

Mr. Tanaka said it was up to the operator to decide what to do with the reactor, because the watchdog did not have the authority to order that it be decommissioned.