Salty water flowing through rocks more than 1 kilometer beneath eastern Virginia came from the Atlantic Ocean when it was much smaller and saltier than today, a new study suggests. Researchers drilled samples at sites along the eastern shore of the Chesapeake Bay, analyzing water that had been trapped in the rocks as much as 1.7 kilometers below the surface. From the concentrations of helium dissolved in that water (bubbles, image), as well as the types of microfossils in the rocks, the team estimated the sediments had been laid down offshore of an ancient coastline between 100 million and 145 million years ago. At that time, the nascent North Atlantic was much narrower than it is today and was a largely enclosed basin surrounded by land—which, along with the warmer climate of the time, helps explain why the long-trapped water is almost twice as salty as today’s seawater , the researchers report online today in Nature. Geologists have long been interested in the area because an asteroid slammed into the Chesapeake Bay about 35 million years ago, blasting a more-than-80-kilometer-wide crater. Despite that crust-shattering impact, the rocks more than 1 kilometer below the surface still retain their original complement of ancient salt water, the scientists say. The water trapped in rocks at shallower depths is less salty than it once was—but still saltier than today’s oceans—thanks to infiltration of fresher water from layers closer to Earth’s surface.