Lake Vostok is under the ice, not under rock. and there are easy to understand reasons for it being there, as for the other lakes in Antarctica. It is the same as any water under ice anywhere. It is three times the volume of Lake Ontario, and about the same in area. This is large for a lake, but not by comparison with the oceans. Underground lakes would have shown up by the way that earthquake waves pass through different materials, the way that the structure of the Earth has been determined. They haven't. We know about the shape of the solid core (it isn't smooth), and about the slabs of rock descending into the mantle from subduction zones along the trenches in the oceans, and about the semifluid layers on which the plates slide. But no-one has found evidence for buried lakes, and there would need to be many of them, and very large in order to inundate all the Earth. Scale is important.

Penny