Scientists have discovered ocean water that dates to the time of dinosaurs trapped more than half a mile underneath the Chesapeake Bay.

Scientists believe the water, which they estimate to be between 100 million to 145 million years old, was trapped when a meteorite or comet hit the Earth near what is now the mouth of the Bay.

It is the oldest sizable body of seawater to be identified in the world, providing a unique window on the past.

“Nobody has been able to isolate an area of water and say this is actually from this ocean at this time,” said Ward Sanford, a scientist with the U.S. Geological Survey.

Sanford said the water was likely in underground aquifers connected to a smaller North Atlantic Ocean that existed in the Early Cretaceous period. It remained stationary in aquifers as they became increasingly buried by sediment, even as the ancient water body was growing to the size of today’s Atlantic Ocean. Then, when the meteorite hit around 35 million years ago, it broke up the rock and effectively sealed off the water.

Scientists discovered the water, which is twice as salty as today’s ocean water, about 1,000 meters below the surface when drilling near the center of the crater.

“We were a little surprised there was that much saline water and the salinity was quite that high,” Sanford said. “We had seen some evidence for it before, but not this high and this much.”

Sanford said ocean water at the time the meteorite hit was not that salty, so they originally thought it wasn’t ocean water. But a chemical analysis led them to believe it was — but not from the large oceans found today.

Using helium dating techniques, the research team concluded that the water was actually much older — 100 million to 145 million years old. At that time, Sanford said, a smaller North Atlantic Ocean or sea existed as a closed basin that was separated from the rest of the ocean.

That isolation would have allowed it to develop higher salinities, like the Dead Sea or Great Salt Lake today. Sanford said the sea was also a little farther south and the climate was warmer, which led to less rain and higher evaporation, and thus higher salinity.

As North America and Africa drifted apart, the water body was opened to the rest of the ocean, and its high salinities were diluted.

No one knows exactly when the salinity dropped to modern levels, Sanford said, “but clearly there was an extended period of time where the salinity on this side of the ocean was substantially different from the present.”

The exact size of the water pocket is unknown, Sanford said, because scientists only drilled one well in that area. But he estimated the volume to be roughly 3 trillion gallons. In comparison, the Chesapeake Bay has a volume of 18 trillion gallons.

The findings stemmed from research on the 56-mile-wide crater that lies beneath the lower Chesapeake Bay. It is the largest crater discovered in the United States, but is now buried beneath 400–1,200 feet of silt, sand and clay.

It’s uncertain exactly where the bottom of the crater is. Sanford said even when scientists stopped drilling at 1,760 meters, they had not reached it.

The results of the study were published in the Nov. 14 issue of the journal Nature.