I knew it! In Journey to the Center of the Earth, Jules Verne's characters go down through endless caves and finally come to a huge underground sea (which, remarkably, they can see to navigate around...). Well, in today's Yahoo News, we see an article entitled Huge 'ocean' discovered inside earth. There is apparently a huge body of water deep in the earth's mantle, under east Asia. In fact, it appears to be centered under Beijing.
They don't say anything about the giant whirlpool Verne described in the middle of his underground ocean.