PLAINVIEW, N.Y., May 21 — When Paul Granger, the water district superintendent here, came to work one morning in 2000, he spotted a rig test-drilling for pollution at a gasoline station across the road from two wells that pump up to 1.7 million gallons of drinking water a day.

He expressed concern that pollution might be threatening the water supply, and eventually his district sued three filling stations, affiliated with Exxon, Shell and Gulf.

As the trial in that case opened in Garden City on Monday, the nation’s water supply industry and major oil companies were watching closely.

The outcome of the case could set a national precedent on who will pay the estimated tens of billions of dollars to clean up contamination caused by MTBE, a potentially carcinogenic fuel additive, now widely banned, that seeped into the ground as gasoline leaked from fuel storage tanks across the country.

It seems an environmentalist's dream: a system that can cool 10 million square feet of Cornell University dormitories, laboratories and computer rooms simply by pumping frigid water from the depths of a nearby lake. No more chlorofluorocarbons, the refrigerants that can destroy protective ozone in the atmosphere. An electric bill 80 percent smaller than for conventional air conditioners. To top it off, nothing goes back into the lake, except water that came from the lake in the first place.

HAVE you ever wondered about health hazards lurking underground near your home, your workplace or a property that you are thinking of buying or renting?

For locations in New York State, there is now an easy way to find out, without resorting to costly testing of groundwater and soil core samples. A free Web site enables anyone — including prospective buyers and sellers, brokers and neighbors — to check a location by typing in its address.

The site responds with an interactive map pinpointing any nearby documented hazards — and there are thousands of them across the state.

BAY SHORE

GLANCING out her home-office window here in 1999, Janine C. DiNatale was puzzled to see a stranger setting up a table on the sidewalk. She went outside and saw two other men, who were wearing hoods and full-body protective gear and were probing the ground.

When several major oil companies agreed earlier this month to pay nearly $424 million to settle a federal lawsuit brought by scores of water providers claiming damages from the gasoline additive M.T.B.E., one Long Island provider took the largest share by far.

The Suffolk County Water Authority in Oakdale, which supplies water to more than 1.1 million customers in the county, walked away with $73.4 million of the settlement. That figure was by far the highest among the Long Island providers and the other more than 150 water companies from 17 states.

Correction Appended

THE water that fills the drinking glasses and bathtubs of Long Islanders comes from right beneath their feet. Thousands of public and private water wells wick groundwater from aquifers, the sole source of drinking water for 2.7 million people.

But a new study shows that they could be getting more than just water.

A four-year federally financed survey of 52 gas stations across Long Island found 32 of them to have previously unidentified petroleum spills that could threaten the Island’s aquifers.

Thousands of years ago, rain fell on Long Island and seeped hundreds of feet through the sandy soil, coming to rest on bedrock. It formed what geologists call the Lloyd aquifer, the island’s oldest, deepest, purest — and scarcest — groundwater.

Now, after 60 years of virtually unchecked suburban growth and consumption of the island’s most precious resource, public officials and civic groups are fighting over control of the remaining water supply. It is as if these were the island’s last drops to drink, which is precisely what environmentalists insist the aquifer should be reserved for.

WITH satellite photographs all over the Internet, images of Long Island as seen from above are only a few mouse clicks away. Views beneath the Island's surface and a clear picture of what may be seeping up and down are not so easy to come by.

The need for a better view of what is going on below ground has assumed new urgency in New York as environmental officials move to evaluate the threat of volatile chemical vapors rising into homes and businesses from contaminated soil and water — some at sites the state said had already been cleaned up.

Tucked away in the woods off a winding side road here, the Lawrence Aviation Industries plant has kept a very low profile since it was established in the 1950's. All that most locals ever saw of the place was the gate and guardhouse just beyond a private railroad crossing along Sheep Pasture Road.

And no wonder: Lawrence Aviation made titanium parts for advanced military aircraft like the Grumman F-14 fighter jet.

Environmental officials say that it also made a terrible mess.

PAUL GRANGER, the superintendent of the Plainview Water District, had to shout over the sound of a 150-horsepower pump as it sucked 995 gallons of water a minute up from a layer of sandy soil 400 feet underground.

"A lot of people don't realize that their water comes out of the ground right underneath them," Mr. Granger said, standing inside the small brick building that houses the pump on the front lawn of the district's headquarters on Manetto Hill Road. That underground water supply, or aquifer, supplies the drinking water for nearly three million people living on Long Island.