The lower flanks of Arsia Mons, one of the giant volcanoes on Mars, is riddled with pits of various sizes. These have been of great interest to a number of researchers because they have the potential to be openings into underground caverns.
Such caverns have some potential for holding ice and offer protection from radiation and small meteorites. These are good things if one is looking for a safe place for current life on Mars or future human life.
However, this image shows some of the pitfalls of such hopes. Most of these pits are largely filled with rubble and dust that hide any potential links to larger underground areas. Furthermore, the entrances are steep and rocky, making them difficult areas to traverse. They would be a very exciting, if not safe, place to visit!
Photo credit: NASA/JPL/University of Arizona
Note: This pit is located on the northwest flank of Arsia Mons.