A new paper in Geology by staff at the NASA-affiliated Lunar and Planetary Institute suggests that Mons Olympos, the 15-mile-high Martian mountain, could only be the odd shape it is if part of it was made up of clay sediments. It may even still contain underground water. And it is a volcano, which means there will be heat. This may make it a very good place to look for life. More details here, and the full paper here.