According to an article in New Scientist, “Mars may have a frozen underground sea”:http://www.newscientist.com/article.ns?id=dn7039 right near the equator. If it’s there, it’s just under the surface, is about 45m (145ft) deep, and covers thousands of square kilometers, about the size of the North Sea.
In related news, I just finished reading “Red Thunder”:http://www.strangehorizons.com/2003/20030929/red_thunder.shtml, John Varley’s recent novel about plucky youngsters launching a mission to Mars. It’s a fantastic read, the kind of book that would be written by the mutant prodigy love-child of Varley, Robert Heinlein, Carl Hiaasen, Spider Robinson, Douglas Coupland, Neal Stephenson, and Carl Sagan.