NASA has just published a bunch of papers that report various findings about the Martian soil made by its robotic planetary rovers. The most amazing, and possibly very important, finding is that about 2% of the soil by mass is actually water. Yup, plain old H2O is just sitting around on Mars... by the megaton.
The water is not necessarily floating about as a free liquid, mind you, because it's incorporated into mineral molecules that are part of the Martian regolith. This is another sign that once Mars may have been covered at least in part by ancient seas, and that the water has merely penetrated beneath the surface or been bound up in chemical reactions.
Why is this so important? In part it will play into the search for alien life on Mars, but actually it's more critical for future manned missions to the red planet. Though they'll have to do a bit of physics and chemistry to access it, the fact that a cubic foot of Martian soil could contain a couple pints of water means astronauts could serve their hydration needs from locally sourced water, rather than having to haul a massive amount from Earth as part of any mission. Now all NASA has to do is crack the tricky space radiation problem.