Las Vegas has pumped so much water out of the ground for drinking and irrigation that the land surface there has dropped four feet in the last 20 years, opening three-foot cracks in outlying areas and posing a potential threat to the hotels, casinos and other big buildings downtown.

The subsidence is most noticeable on the north side of the city, where underground, water-bearing rock strata, known as aquifers, converge. There, cracks can be clearly seen in streets, curbstones and some buildings.

In some cases cracks in the earth have been filled in by developers building houses. One source in the Geological Survey said the developers have described the filling of the cracks as "grading" of the land.

"We're not ready to evacuate downtown Las Vegas," said one source at the United States Geological Survey, "but there is a possibility that the surface fissuring could get worse and might even infiltrate the more populated parts of the city."

Now a city of 165,000 with a metropolitan population of 390,000, Las Vegas has been drawing its water for the last 40 years from a series of wells the wells tap a network of porous rocks deep underground which carry runoff from rain and melting snow in the mountains west and north of the city. Las Vegas uses more than 24 billion gallons of water in a year, all of it from underground sources.

For the last 10 years, the land around the city has fallen at a rate of one tenth of a foot every year. The underground water supplies have themselves fallen between 150 and 200 feet, making it so difficult and expensive to pump the water to the surface that the city recently signed a contract to begin pumping water from the Colorado River.

"There is a direct relationship between the ground subsidence in Las Vegas and the withdrawal of groundwater," said the source at the U.S. Geological Survey."The trouble is it's the sole source of municipal water in Las Vegas."

Three weeks ago, a panel of five geologists convened at the U.S. Geological Survey and took a vote on whether the subsiding ground posed a hazard to the people of Las Vegas.

Two of the panel members said it was a hazard, two said it was not a hazard and the fifth was undecided. Their "concern" was passed on to Survey Director Vincent E. McKelvey, who confirmed that Las Vegas is being watched more closely for signs of stress in the earth.

"The trouble is we don't know enough geologically about this problem," McKelvey said. "What we're worried about is not just the subsidence of the ground but the possibility of seismic movement (earthquakes) as a consequence of groundwater withdrawal."

There are no earthquake zones around Las Vegas the way there are in California, but geologists are concerned that the ground may be falling near a geologic "fault" and that could cause the fault to move. They are concerned that, in effect, a mild earthquake could be triggered by the groundwater withdrawal.