Scientists at the Lunar and Planetary Science Conference in Houston, Texas reported that images from NASA's Odyssey spacecraft show what may appear to be seven cave openings on the flanks of the Arsia Mons volcano. The entrances range in width from 100 to 250 meters. and the caves are somewhere over 70m in depth (in only one case can the floor actually be seen.) The caves offer possible shelter for any Martian life forms and could conceivably be of use to human colonists.
This is a big week on the Red Planet, including as it does the reported discovery of a massive underground "sea" of frozen water ice: