This cavern opening, about 100 meters across, is found on a lava plain in the volcanic Tharsis region of Mars. It’s so dark and deep that even the Mars Reconnaissance Orbiter’s sensitive cameras cannot peer into its depths (although the shot was taken at 03:27 p.m. Mars time, so the sun was about 38 degrees above the horizon). It’s probably a collapse pit formed when a lava tube or similar structure underneath the surface gave way. The entrance walls may actually be overhanging, concealing a space who knows how large that hides who knows what.
https://hirise.lpl.arizona.edu/PSP_003647_1745