A Hole (One of Many) on Mars

Mars is pockmarked with strange, nearly circular holes. The hole in this image was discovered by chance on images of the dusty slopes of Mars' Pavonis Mons volcano taken by the HiRISE instrument aboard the robotic Mars Reconnaissance Orbiter currently circling Mars. The hole appears to be an opening to an underground cavern, partly illuminated from the right. Analysis of this and follow-up images revealed the opening to be about 115 feet (35 meters) across, while the interior shadow angle indicates that the underlying cavern is roughly 66 feet (20 meters) deep. Why there is a circular crater surrounding this hole remains a topic of speculation, as is the full extent of the underlying cavern. Holes such as this are of particular interest because their interior caves are relatively protected from the harsh surface of Mars, making them relatively good candidates to contain Martian life. These pits are therefore prime targets for possible future spacecraft, robots, and even human explorers.