NASA Mars Orbiter Spots Caves on Red Planet
Huge pits or caves pumcture a bright, dusty plain near the Martian volcano Ascraeus Mons in an image taken between October 1 and November 1 by NASA's Mars Reconnaissance Orbiter (MRO). Caves and other underground structures, including lava tubes, and canyon overhangs would be potentially useful for manned missions to the Red Planet, providing shielding from both the elements and intense radiation that a Mars mission would expose astronauts to. They might also offer access to minerals, gases, ices, and any subterranean life that the crew of such a mission would probably be searching for.
More info: NASA