Scientists say they have uncovered the oldest body of seawater in the world under Chesapeake Bay.

The seawater, found 0.6 miles underground, is most likely 100 to 145 million years old, and a remnant of the Early Cretaceous North Atlantic Sea. The findings, published in the journal Nature, supports the theory that the North Atlantic Ocean’s transition from a closed basin to an open portion of ocean caused saline levels to decrease.

“What we essentially discovered was trapped water that’s twice the salinity of [modern] seawater,” Ward Sanford, a U.S. Geological Society (USGS) hydrologist, told the Washington Post. “In our attempt to find out the origin, we found it was Early Cretaceous seawater. It’s really water that’s from the North Atlantic.”

The water was found inside the largest crater in the United States. Believed to be created by a huge rock or chunk of ice that slammed into an ancient ocean 35 million years ago, the crater was hidden from sight under 400 to 1,200 feet of sand after it caused gigantic tsunamis that reached as far as the Blue Ridge Mountains, 110 miles away.

Using water samples pumped from the site in 2005, scientists were able to determine the salinity of the water was twice of the modern Atlantic Ocean. They did so by analyzing the water’s chemistry where they discovered high levels of chlorides and bromides -- a signature of ancient seawater.

"We knew from previous observations that there is deep groundwater in quite a few areas in the Atlantic Coastal Plain around the Chesapeake Bay that have salinities higher than seawater,” Jerad Bales, acting USGS associate director for water, said in a statement. "Various theories related to the crater impact have been developed to explain the origin of this high salinity. But, up to this point, no one thought that this was North Atlantic Ocean water that had essentially been in place for about 100 million years."

Unlike other studies that have used indirect evidence to determine salinity levels, this is one of the first to use ancient water samples that have remained “in place in its geological setting," which has allowed researchers to give a “direct estimate of its age and salinity,” according to Sanford.

Sanford told the New York Times that there is probably more water trapped under the Chesapeake Bay and other places along the East Coast. It would be costly and difficult to find most of them. “Until someone decides they want to spend that kind of money again, it’s hard to go there to see what else we can learn,” Sanford said.