RETSOF, N.Y. -- In the heart of the Genesee River Valley, at the point that Route 20A meanders across Beards Creek, a gash a foot wide and six or seven feet deep runs through the earth.

It looks like scars from an earthquake. When the ground opened on March 12 and tremors were measured at 3.6 on the Richter scale, that's what people here thought it was. But that was before the seismic data came back negative, and local officials and residents slowly realized that the disaster that has shaken the Genesee Valley was of the distinctly unnatural kind.

Retsof is home to the largest salt mine in North America. It covers an area the size of Manhattan, 1,000 feet beneath the rolling farmland of this valley 25 miles south of Rochester. On the morning of March 12 the southern tip of the mine suddenly and dramatically caved in, the result, some residents and experts say, of questionable extraction practices by the mine's owner, Akzo Salt Inc.

No one was hurt in the accident, and there were no houses on the site of the collapse, only three unoccupied tractor-trailers. But during the weeks since then, ground water and water from subterranean aquifers has begun to pour into the mine through the cracks created by the cave-in, filling the giant cavity at a rate of tens of thousands of gallons a minute. Early this month the company scuttled its attempt to pump out the onrushing water when the pumping could no longer keep pace. Engineers hired by the company will, this week, begin the more difficult task of trying to find and plug the leaks.

If the repairs cannot be completed, the country's leading source of road salt -- the mine that supplied a convoy of trucks in the Washington area with salt last December, during one of the winter's innumerable snow storms -- will be underwater completely by early next year. The mine management is preparing to deal with future demand for salt, but a flooded mine would portend untold implications for the area's ecology.

"We're watching one disaster after another," said Melissa Jacobs, whose husband works in the mine. "We've lost a 100-year-old historic salt mine. We're going to lose farmland because of the lack of ground water. They've just changed the geology of the whole Genesee Valley."

In the past month, relations between the Akzo firm and the community have deteriorated. One town meeting after another has ended in acrimony. Rumors among local residents allege everything from a coverup by the company of warning signs in the weeks before the accident to wild accusations that the company staged the accident to win support for its controversial plan to use the empty parts of the mine to store incinerator waste.

The company denies those charges. But the state attorney general's office has begun to ask questions in the area, and some of the 300 miners employed by the company have begun to worry about their jobs.

"We are just at the beginning of seeing what the problems are going to be," said Barry Caplan, a nearby resident, who, like many, worries that shifting ground will imperil the houses of the homeowners above the mine. "It's a great big experiment."

At the source of the controversy over the cave-in is a mining technique that Akzo began using several years ago in the southern part of the mine, directly under where the collapse of the mine roof occurred.

For much of the mine's history, miners left untouched about half of the rock salt they encountered in huge, 75-foot-square pillars to support the roof of the mine. The result was a kind of eerie underground cloister: Cavernous rock rooms, large enough to drive a truck through, run for miles underground. Just before the collapse, producers of the Hollywood film "Die Hard 3" had expressed interest in filming a chase scene in the Retsof mine.

Two years ago, however, the company began experimenting with a different technique. Around the sides of these giant underground rooms, it would leave huge walls of undisturbed rock. But in the 300- by-1,200-foot rooms themselves, as much as 90 percent of the salt would be removed, leaving interior pillars as small as 20 feet across to hold up the ceiling.

This technique has been successfully tried in other salt mines and is supposed to make for a safer mine. "The object is to take the stress off the areas you are actively mining and move it over onto those huge pillars to prevent the roof from caving in," said Dick Thompson, vice president of Akzo Salt. "Ironically, it's supposed to prevent the exact problem that we've got."

After some initial success, however, late last year the two small rooms where Akzo was trying out the new technique began to show signs of instability. Three months later, the mine suffered a cave-in immediately above the two rooms where the experimental mining technique had been tried.

Akzo officials say they still don't believe the small-pillar method is at fault. Thompson said they are looking closely at the layer of limestone immediately above the salt mine, for evidence that it may have had serious cracks that might have caused the cave-in.

"We're not using a different technique than {other mines} are using, and they've never had these problems," said company spokeswoman Molly Mangan.

"That's what makes us think that there is something else going on. ... I'll be honest with you. I don't know if we'll ever know the cause of the collapse."

For others, however, the cause is clear. "The problem was over-extraction," said Andrew Michalski, a New Jersey-based mining engineer, who has done consulting work with local environmental groups. "Too much salt was taken away. ... Those little pillars served no useful purpose. They were too small to provide any meaningful support."

Michalski, in fact, speculates along with many local environmentalists that the root of the problem was Akzo's proposal to regulators last year to be allowed to use the empty parts of the mine as a storage facility for ash leftover from municipal incinerators.

"I think they got greedy and wanted to make more room for the ash," he said.

But Akzo officials dismiss such charges; the experimental technique doesn't result in any more space being cleared underground, but simply creates a different configuration of pillars, they say.

Early this month, the Supreme Court ruled that incinerator ash was to be classified as hazardous waste, a ruling that Akzo officials concede vastly complicates their plan. More to the point, the safe dry place that the company had been advertising as a storage space is now filling with water, at a rate of somewhere around 20,000 gallons per minute.

This week the company will start a concerted effort to plug the leaks. But few are optimistic. The procedure has rarely been successful with leaks this large. Further, the "grouting" process, as it is known, requires drilling holes at random down into the collapsed area, in the hopes of happening upon one of the leaks and then stopping it up with concrete.

"It's a very tough problem," said Lawrence Lungrun, a geologist at the University of Rochester. "It's sort of like finding a needle in a haystack."

While the waters rise, Akzo has put its miners to work in the part of the mine that is still dry. There is talk of opening another shaft south of the current mine and, if worse came to worst, of shipping in salt from the Caribbean to meet future demand for road salt.

Residents of the area, meanwhile, are waiting. The good news, according to geologists, is that if the mine totally fills with water, the resulting thick, salty brine will form an effective filler for the underground caverns, making further cave-ins unlikely. The bad news, however, is that the ground is likely to steadily settle over the next several years, taking the foundations of houses with it; and the availability of ground water is likely to be affected.

Local homeowners are already worried about whether anyone would want to buy a house above the world's largest underwater salt mine. "It sort of feels like Love Canal," said Caplan.

The collapse of the largest salt mine in North America may have implications for the ecology of New York's Genesse River Valley.

For the past two months, ground water and water from subterranean aquifers have poured into the Retsof mine. Two weeks ago, the mining company abandoned pumping.

1. March 12: Southern tip of Retsof salt mine collapses.

2. Sinkhole appears at surface above mine, Route 20A bridge closed.

3. Local residents in towns above 11-square-mile mine fear ground subsidence over time.

4. Farmers in Genesee River Valley may be affected by lack of fresh ground water.

SOURCE: News reports, New York Bureau of The Washington Post