Was Earth once a water world?

Was the Earth's surface once inundated with water?

Clever new research combining physical models with experiments adds to the growing evidence that billions of years ago the Earth was a water world, covered in twice as much water as it has on the surface now, possibly submerging even the highest peaks.

It's not proof, but things really are starting to line up that this is the case.

The mystery here lies in the Earth's mantle. This is the layer of extremely hot (but not molten!) rock under the...