Mars clearly had a watery past, and it's expected that much of the water is still on the planet. Figuring out where the ice is hiding could tell us a lot about the planet's climate history and something about Mars' current water cycle. It could also help direct future landers to sample the planet's water and possibly use it to support human landings.
While we've found plenty of ice near the pole during the Phoenix Lander mission, that's not a very convenient location for future landings (in part because the site ended up frozen over with dry ice during that pole's Martian winter). In today's issue of Science, researchers are reporting the likely presence of ice sheets in more temperate regions. The sheets are at least 100 meters thick and appear to preserve layers that may help us reconstruct how the water ended up frozen there.
As with many things Martian, the work relies on data from the Mars Reconnaissance Orbiter. It has a variety of instruments that can probe the chemical composition and subsurface structure of Mars, along with the best camera we've ever sent to another planet. Over the years, MRO has built up a comprehensive catalog of features on the Martian surface, many of them imaged from multiple angles.