Look out from their deck and the Chesapeake Bay spreads out in sun-glittered splendor, an azure expanse punctuated by sailboats and seabirds and far in the distance the wooded shoreline of Maryland's Eastern Shore.

Look down from their deck, 100 feet nearly straight down, and try not to lose your lunch.

The spectacular view is what attracted retirees Marcia Seifert and Phyllis Bonfield from Philadelphia to the top of Calvert Cliffs in Southern Maryland five years ago, but they didn't realize it would get quite this dramatic. Their two-story home is now 12 measly feet from the edge of the precipice, about 35 feet closer than it was two years ago.

The inexorable erosion of the cliff face has sloughed off massive slabs of earth and trees, and because an endangered beetle happens to live in the cliffs, the residents have not been allowed to stop it.

"It's scary," said Bonfield, looking down the dizzying sand and clay cliff face.

"We don't know how long Mother Nature will allow us to be here," Seifert said.

Erosion is like receding gums or the depreciation of cars. It is a slow battle and one you most surely will lose. And along the circuitous 7,700 miles of Chesapeake Bay shoreline, the wind and waves are winning in ever more emphatic fashion. Geologists say sea levels in the bay are rising about one to two feet per century -- a rate double the world average -- although they do not agree how much is because of global warming and how much because the land is subsiding.

Add to this storm water rushing down from an increasingly developed coastline, and scientists find that land in many spots is falling into the sea at a rapid clip. Every century, an area roughly the size of the District is being lost around the Chesapeake Bay. Each year, about 260 acres of shoreline disappear from Maryland alone. Populated islands that once speckled the bay have been submerged and swept away.

"When you're raising the water level, you just have so much more susceptibility to any kind of wave attack," said Court Stevenson, a professor at the University of Maryland Center for Environmental Science's Horn Point Laboratory in Cambridge.

In such places as Calvert Cliffs, erosion happens in fits and starts. After the bay licks away the toe of the cliff, a big storm or prolonged rains can saturate the top until it cannot bear its own weight and crashes down.

This month marks the start of hurricane season; two years ago, the surge of Hurricane Isabel tore 20 acres of land from the Chesapeake's western shore. This time around, the cliff dwellers of Chesapeake Ranch Estates said they worry about when their number might be up.

"Calvert County is already the smallest county in Maryland, and it's getting smaller every day because it's falling into the bay," said John Eney, president of Chesapeake Ranch Estates, which has 3,700 homes, including 100 perched on the lip of Calvert Cliffs.

In 1996, an 11-year-old girl walking along the beach was killed when part of the cliff collapsed, homeowners said. Now, signs in Chesapeake Ranch Estates prohibit people from using the beach, although some still risk it.

To try to halt collapses, homeowner Tony Vajda is manufacturing 380 two-ton hollow concrete domes called "reef balls" that he will drop 300 feet offshore in a few weeks in hopes of slowing the waves.

The $200,000 investment, divided among four families, is the final result of eight years of pushing to erect some type of breakwater in front of the cliffs. Earlier ideas, such as a geotextile tube filled with sand, had been rejected by state officials because they impinged on the domain of the puritan tiger beetle, an endangered species that lives hardly anywhere else in the world.

"The beetle relies on natural erosion for its habitat, a little sand kicking down from the top. And if you stabilize the toe of the cliff or top of the cliff, then their habitat goes away," said Greg Bowen, Calvert's director of planning and zoning. "Another conundrum: If they stabilize that cliff face, it could accelerate the erosion just up or down the coast."

The U.S. Army Corps of Engineers approved the reef ball project because it was offshore and is intended to slow erosion, not stop it. But the Corps is requiring that Vajda document shoreline conditions on a quarterly basis and after every major storm for 10 years. Vajda wants to put the reef balls -- designed with holes to create habitat for fish, crabs, mussels and other sea life -- across the entire 2.3-mile expanse of the neighborhood. But so far only the first 500-foot phase has been allowed.

"It's amazing; these people put you through so many hoops," said Vajda, 57, a former program manager with the Army Corps of Engineers who served on a state task force in 2000 to assess the severity of Maryland's erosion problem. His house stands 60 feet from the edge, but a large gouge in the cliff face a few yards to the right of his home already extends back past the line of his patio. "If I see it getting serious, then I'll get serious. I'll put down rock, permit or no permit. Enough is enough."

But Curt Larsen, a research geologist who retired recently from the U.S. Geological Survey and lives and studies in Calvert County, said he believes erosion might prevail, no matter the defense.

"There's not a silver bullet. If you invest megabucks to pile rocks at the bottom of the cliff face . . . then the top of the cliff starts to retreat. The [erosion] process changes," he said. "It changes from the face sloughing off near vertically, to the top retreating backward . . . and the house goes down anyway."

In the next few months, Maryland geologists plan to unveil a Web site called Shorelines Online, which will allow people to see with great detail how the coast has changed over time and help assess their erosion risk. The project digitized navigation charts dating back to 1848 and superimposed them on recent aerial photographs of the entire shoreline. It also allowed state officials to pinpoint erosion hotspots, such as Taylors Island, directly across the Chesapeake Bay from Calvert County, where land is disappearing at a rate of 12 feet per year, said Lamere Hennessee, a researcher at the Maryland Geological Survey.

Bruce Coulson's property on Taylors Island is deeded at 15 acres, but only 10.8 remain. Over his 18 years there, he's watched the water erode through hundreds of feet of forest on property south of his.

"You can walk out to your knees in water, reach down and what you pull up is tree bark. It's laying all over the bottom," said Coulson, 56, who runs a campground on the island and has fortified his land with about 100 two-ton concrete blocks. "Everybody's telling me I'm going to have my own island all around me."

To protect their Calvert Cliffs property, Seifert and Bonfield have regraded their back yard, moved the septic tank and diverted all pipes from the edge, in an effort to keep storm water off the cliff. Under legislation sponsored by Del. Anthony J. O'Donnell (R-Calvert) that passed this session and is now law, beetles can be "taken" under certain circumstances to protect property, and the women have applied for an emergency permit to fortify their cliff. They have yet to hear back from the state.

This season, if a hurricane is predicted, they say they will evacuate temporarily. But they've decided to fight erosion rather than move.

"It's a million-dollar view," Seifert said. "And when the house slides into the bay, oh well."

As Chesapeake Bay water levels continue to rise, residents of such communities as Chesapeake Ranch Estates fear for their homes.Marcia Seifert and Phyllis Bonfield's home is about 35 feet closer to the edge of Calvert Cliffs than it was two years ago, thanks to erosion.Tony Vajda is manufacturing hollow concrete domes that he will drop offshore in hopes of slowing erosion. The $200,000 cost will be divided among four families.