A NASA orbiter found evidence of a vast frozen sea lying just below the powdery surface of Mars’ southern hemisphere, an icy expanse that could extend from a few inches below the planet’s surface to hundreds of meters deep. The discovery hints at the potential for life in the planet’s past–or even present–and raises the stakes for future Mars missions by offering the promise of cheap water for cooking, hydroponic agriculture and make-it-yourself rocket fuel, all key variables in proposed manned expeditions to Mars. It also appears to answer a mystery that has been posed with every Martian photograph showing dry rampart craters, river outwashes and ancient canyons: What happened to the water? Apparently, a good deal of it is still there.