The Pavonis Mons volcano contains a 115-foot-wide "skylight" opening to a cavern below, as seen in this August 17 picture from NASA's Mars Reconnaissance Orbiter. Caves often form in volcanic regions when lava solidifies on the surface but keeps flowing underneath. The underground rivers of lava drain away, leaving an empty tube. The skylight forms when a section of crust falls into the tube below. Based on the shadow cast on the floor of the pit, scientists estimate that the cavern is about 55 feet deep. These caverns could shelter astronauts during future missions to Mars.