Pavonis Mons Skylight
Lava tubes have long been considered an ideal spot for potential underground bases on both the moon and Mars.
The reasons are pretty clear: protection from radiation, less work and effort to excavate, ease of access...
And a telling sign of lava tubes that might be worth exploring are the 'skylights' entrances -- collapsed portions of the tube that present a 'window' into the underground structure.
This image is of one of those 'skylights'.
Taken by NASA's Mars Reconnaissance Orbiter in 2011, this shot is from the western slopes of Pavonis Mons at the bottom of a much larger 'crater-like' feature of unknown origin. Pavonis Mons is an ancient shield volcano in the Tharsis bulge region just west of the Valles Marineris and is the middle of the three volcanoes that make up the Tharsis Montes: from South to North they are Arsia Mons, Pavonis Mons, and Ascraeus Mons.
The image clearly shows a collapsed entry into a possible lava tube that is absolutely stunning! The entry is about 35 meters in diameter (about 115 feet across) and 20 meters deep (about 65 feet), and, if you look hard enough, you can see the pile of rubble that has fallen down the steep slopes and into the cave entrance.
If only we could peek inside!