Sunlight casts a crescent shadow on the floor of an underground cavern in a recently released picture of a Martian "skylight." This round opening likely leads to a lava tube, which is created when lava solidifies on the surface but keeps flowing underground. Eventually the lava drains away, leaving an empty cave.
Snapped by NASA's Mars Reconnaissance Orbiter, the picture revealed that the skylight is 115 feet (35 meters) across. The shadow helped scientists calculate that the cave floor is about 65 feet (20 meters) below the surface.