Today's Image of Mars shows what is very likely the entrance to an underground cavern on Mars. The cavern is located Northeast of Arsia Mons and can be seen at the top of this image.
Cavern entrances like this one are called skylights. They have no raised rim or ejecta, which differentiates them from impact craters. This skylight is so special because even the extremely sensitive HiRISE camera cannot detect what lies within the shadow if the skylight. In fact, HiRISE cannot see inside the shadows of this pit at all; it can't even see the walls of the pit.
Scientists speculate that this skylight must have overhanging or completely vertical walls, which brings them to the conclusion that this is a collapse pit into an underground cavern. This pit must be very deep for sunlight not to illuminate the bottom.
In recent years it has been suggested that underground caverns like this one are probably much more suitable to life than the surface, which has been irradiated and is hostile to life as we know it. Caverns like the one below are likely to be prime targets for future manned and unmanned missions to Mars! Some scientists even speculate that colonies could be set up inside of caverns because they act as a natural shield to space radiation and the harsh Martian environment.
Clicking on this image will take you to the original, captioned image from HiRISE.