The water in the mantle rock, which could equal the amount of water in the world’s oceans, may be an integral part of sustaining water on the surface, Smyth said. It also implies that the Earth’s oceans likely formed from water stored deep beneath the Earth, rather than from a comet or asteroid, as other scientists have theorized, Smyth said.

“Geophysicists have debated where…Earth’s water came from,” Jacobsen said. “From inside the Earth out of rocks as the planet formed, or did Earth’s water come from things like comets much later on? I think this finding is supporting the idea that oceans and the atmosphere came from (inside the Earth).”

via Water discovered deep beneath Earth’s surface.