One of the worst cosmic collisions known to have rocked what is now the United States carved a huge crater from the present-day mouth of Chesapeake Bay.

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The asteroid or comet impact kicked a cloud of debris high into the atmosphere, spawned devastating tsunami waves up to 2,000 feet (610 meters) high, and carved out the largest crater ever found in the United States, researchers say.

So this week, scientists with the U.S. Geological Survey (USGS) planned to begin drilling a 1,500-foot (457-meter) hole in the ground near the outer rim of the now-buried crater. The drilling is part of an ongoing project to help local water companies manage a deteriorating supply. Researchers have learned that the rim is a tenuous boundary between salty groundwater within the crater's confines and fresh groundwater on the outside.

Last summer drilling in a separate hole at NASA Langley Research Center, which sits on the crater rim in Hampton, Virginia, turned up quartz crystals that researchers say can only be caused by cosmic impacts.
 
 

The crater

The water woes started when a huge object slammed into Earth 35 million years ago. The impact left a now-buried, unstable crater rim that still generates earthquakes as it shifts and sloughs around.

"The asteroid or comet probably measured about 1 to 2 miles (1.6 to 3.2 kilometers) in diameter and was traveling at tens of miles per second," said Greg Gohn, USGS Chief of the Chesapeake Bay Impact Crater Project. "It gouged a crater 53 miles (85 kilometers) wide and fractured bedrock to a depth of well over a mile. Today, those disrupted rock units greatly affect the pattern of groundwater flow throughout southeastern Virginia."

Gohn said last week that the continued drilling will help researchers manage groundwater resources in southeastern Virginia.

The impact created a two-tiered depression, like an inverted sombrero, in what was then a shallow part of the ocean, explains David Powars, a hydrologist with the U.S. Geological Survey.

Over time, rivers in the region turned to flow into the crater before going out to sea which, along with glacial advances and retreats, carved out Chesapeake Bay, Powars said. The bay, he said, is no more than a drowned river system.

Next page: A giant tsunami

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"It's an incredible collision that we're talking about," Powars said in 1999 after the crater was confirmed to have been caused by an impact. He likened it to putting all the world's nuclear weapons in one spot and detonating them simultaneously. "The force of the impact ejected huge amounts of debris into the atmosphere and spawned a train of gigantic tsunamis that probably reached as far as the Blue Ridge Mountains."

The crater is buried under 400 to 1,200 feet (120 to 365 meters) of sand, silt and clay. Its existence was originally suspected in 1993 after Powars and colleagues studied oil company seismic data.

Powars said the incoming projectile lifted the ocean floor as much as 200 feet (60 meters). Like a giant paddle, this would have sent huge waves traveling outward in all directions and back and forth, carving a crater that is much different than the more evenly sculpted variety caused by impacts on land.
 
 

"Because of the giant holeyou could have had water from the ocean slosh in and slosh out," Powars said, explaining that the waves created a jagged and irregular outer rim that jumps in and out a mile or so in various spots.

"You have a real mess in there," Powars said.

Ancient marine fossils and other debris sloshed in and are now buried deep inside the former hole. What isn't in there is a whole lot of debris from the object that caused the whole mess. Most of it seems to have vaporized.

Drinking water still affected

The irregular, unstable outer crater rim -- which extends 10 to 20 miles (16 to 32 kilometers) inland in Virginia -- appears to answer several questions about unusual phenomena in the region, including salty groundwater and earthquakes around the crater's perimeter.

Powars said the rim's instability, caused when the underlying crust was banged up during the impact, may explain the high seismic activity in the region. The rim is constantly shuffling and settling, triggering seismic waves.

The rim is also a boundary between salty groundwater within the crater's confines and fresh groundwater on the outside. Powars said about 2 million people count on the region's groundwater, and experts have long worried that frequent intrusions of saltwater into the drinking water were caused by over pumping. Powars said the new explanation has helped utility companies dig wells outside the crater rim to more successfully search for potable water.

Gohn expects to begin drilling at two sites in Virginia. Drilling of a 1,500-foot hole just inside the crater rim in Mathews County, Virginia was expected to begin March 20. Work at a second site near Shadow, Virginia is planned to begin in June. This site is located well within the ring-shaped trough of the impact crater, and scientists expect to penetrate the bottom of the crater at a depth between 2,000 to 2,500 feet (610 and 760 meters).

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