credit NASA/JPL/University of Arizona
The surface of Mars is not completely flat, nor is it completely solid. Long extinct volcanoes leave lava tubes behind as they eventually fade away. These tubes will sometimes open up on the surface, creating what is termed a skylight.
The skylight pictured was possibly opened up by a meteor striking the ground, leaving behind the hole into the tube. This crater is about 600 feet (175 meters) across, with the hole itself about 115 feet (35 meters). This picture is interesting because it even shows the lava tube floor itself, about 60 feet (20 meters) below.
It might be that these skylights may prove to be an important tool for future explorers. The tubes could be used as subterranean bases, with the surface providing protection for the colony. It's almost as if Mars was trying to be accommodating.
