The surface of the earth is mostly covered by water, but below the surface, it is mostly hard stuff like rock, molten rock and iron. Volcanoes are holes on the surface that allows what is under the earth to come out, and that stuff, called lava, is definitely not water. The earth's center is 3,958 miles below the surface, but the deepest part of any ocean is only 124 miles deep. That means there are at least 3,834 miles of solid stuff below even the deepest ocean. That means if you start drilling at the bottom of one ocean, you will not come out at the other end until you have drilled 7,600 miles through solid rock, molten rock and molten metal.