The southern pole of Mars is hidden beneath a "deep and wide" layer of ice - enough that if it melted*, it would cover the whole planet in a sea 36 feet deep**. Shallow for a sea, but still a fair quantity of aqua.
The findings are published in the 15 March online edition of the journal Science. Lead author Jeffrey Plaut said: "The south polar layered deposits of Mars cover an area bigger than Texas. The amount of water they contain has been estimated before, but never with the level of confidence this radar makes possible."
More: http://www.theregister.co.uk/2007/03/16/icy_mars/
(* Not on our list of likely events, either. -- ElReg)
(** I assume this statement's ignoring the effects of topography, or something daft like that -- Rick)