Where did all the H2O go? Meteorites resembling the raw material from which Earth formed contain 2 percent water, yet today's oceans make up only about 0.02 percent of our planet's mass. Kei Hirose at the Tokyo Institute of Technology thinks some of it is locked away deep beneath our feet. He and his colleagues simulated conditions in the lower mantle, 400 to 1,800 miles below the surface, by heating minerals to 2,900 degrees Fahrenheit and squeezing them to 250,000 atmospheres in a diamond anvil. Spectro-scopic analysis revealed that the baked rocks held on to a surprising amount of hydrogen, suggesting that mantle rocks may contain a vast amount of water—five times as much as in the oceans. The water in those rocks may help keep them pliable, facilitating Earth's internal churning—the process that ultimately causes earthquakes and continental drift. When sections of crust sink under the oceans, they probably drag additional water with them. "We believe water is being transported into the lower mantle," says Hirose. At the pace predicted by his model, the oceans may dry up in a billion years. This result may also explain the fate of the water that carved out surface features on Mars billions of years ago.