Devin Galloway, a USGS scientist who specializes in subsidence but did not participate in the new study, said he was surprised by the extent of both seasonal and long-term movement caused by groundwater siphoning. Galloway said the study, which he has read, was based on solid methods and good data.

"Water managers should be aware of it," Galloway said. "But subsidence is one of those issues that's really hard to get on peoples' radar screens. It's so subtle, that people don't really notice it unless something happens."

They may notice before too long.

"The city of Santa Ana [just south of L.A. proper] is sinking at the rate of about a half-inch a year," Bawden said. California's huge system of aqueducts, used to move water into the region from Northern California and from the Colorado River, currently slopes just 4 inches per mile to provide enough grade to keep water flowing.

"It's not going to take too many years before water is not flowing the proper direction," he said.

But sewer pipes typically rely on gravity alone, and they may soon be fighting an uphill battle to get waste out of sunken areas. In the more distant future, a depression could form that would cause portions of the Santa Ana river to become a lake, Bawden said.

If it comes to that, no one can say there were no warnings. As early as the mid-1950s, geologists noted that groundwater pumping near the Santa Ana river had caused a primary aquifer to drop below sea level, and saltwater had flowed into the aquifer up to 5 miles inland.

To fight this problem, a series of 23 wells were drilled so that officials could pump up to 1.3 million gallons of water into the ground each day. The effort creates a hydraulic wall of sorts that keeps seawater at bay.

Subsidence caused by groundwater pumping is nothing new. One region of California's Central Valley, well north of the Los Angles area, is known to have subsided roughly 30 feet (9 meters) since the late 1940s. Other parts of the country are also affected.

In 1997, the USGS measured long-term sinking in select locations in or near these cities:

• West of Phoenix, Arizona: 18 feet (5.5 meters)
• Houston, Texas: 9 feet (2.7 meters)
• Las Vegas, Nevada: 6 feet (1.8 meters)

Measurable side effects have already occurred. At Edwards Air Force Base in California, a giant crack in the ground caused by groundwater siphoning once rendered a backup runway designed for the space shuttle unusable.

Bob Pierotti saw that huge fissure back in the early 1990s. He called it "the mother of all fissures, a big gaping crack" that was some 20 feet wide and hundreds of yards long. Pierotti is a geologist with the southern division of the California Department of Water Resources. He says that in general, subsidence is viewed seriously by those who manage water districts.

He said studies like the new one provide valuable information for water managers charged with planning current and future water storage practices.

But addressing subsidence in the Santa Ana basin would require the cooperation of many agencies. There are several water districts in the greater Los Angeles area, some operating locally and some regionally, with multiple layers of bureaucracy.

Subsidence is not confined to the West or to the areas studied so far. It could be happening under your feet, too, geologists say, especially if you live above an aquifer and near a highly populated area or in an agricultural region. Much of southern and coastal New Jersey, for example, sits above a large underground water system.

Wells have pumped the New Jersey groundwater to the point that seawater backs into the aquifer, rendering the water undrinkable in many low-lying places far inland.

Such thirst is a growing problem around the country.

Since the mid-1950's, the amount of water in the United States supplied by groundwater pumping has grown from less than 20 percent of the total to more than 30 percent. Much of this rise occurred in the Southwest, where the population burgeoned after World War II. Officials expect the problem to grow as more rivers and lakes are tapped out.

But in many parts of the country, the effect is hard to spot. Bawden and his colleagues used satellites to study ground movement over time by bouncing radar off the surface on subsequent passes in the satellite's orbit. The technique does not work well in heavily forested areas because the radar bounces off tree leaves instead of the ground.

For Californians, as well as lovers of Mickey Mouse, there is some comforting news.

For locals who worry that drinking tap water might actually help bring on the next Big One, Bawden and his colleagues did a little investigating: They took a close at one active fault and determined that the human activity "has no bearing" on the current seismic activity of the fault.

Nor is Disneyland in any imminent danger. The annual rise and fall is over such a broad area, Bawden said, that it doesn't threaten to cause any structural damage at the park.