It's not that all those probes and rovers have been looking in the wrong place. The polar ice caps aren't part of the search range for signs of Martian water ('cause, doi, they're ice caps). But recent estimates by scientists from a joint-NASA Italian Space Agency experiment show that there's even more water than previously thought.

There's more ice down south than previously thought. Enough to cover Mars with a layer of water approximately 11 meters/36 feet deep. Hope you guys brought some swim bladders.

Says Jet Propulsion Laboratory's Jeffrey Plaut, "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." These calculations were conducted with readings off the European Space Agency's Mars Express' radar system, the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS).

This discovery is part of the research into the geological (and potential biological) history of the Red Planet, however. Along with those aquifers and the occasional breakout of Martian acne water, researchers try to discover if Mars had ever supported life in the past - and how all that water ultimately retreated underground and to the poles.