While some think of underground seas when considering the moon's of Jupiter there are many on earth. It has been estimated that there are over 145 lakes underneath Antarctica's snows. America has The Lost Sea which is a 13 acre cave system completely filled with water.
The idea of an underground body of water is not new. Many ancient mythologies like Egyptians and Greeks had seas and rivers below the surface. Even the drugged out Samuel Coleridge dreamt of a sunless sea in his "Kubla Khan" poem. However, one must remember the "ocean" is really just wet rocks before one makes plan to buy a submarine.