The Mars Reconnaissance Orbiter, which is currently circling that planet, has produced a stunning photo that shows a hole on the dusty slopes of the Pavonis Mons volcano.
The image, taken by the Orbiter's HiRISE instrument, shows a hole about 35 meters across. A visible shadow shows an underlying cavern believed to be about 20 meters deep.
As NASA explains, "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 interplanetary explorers."