Pavonis Mons, Mars. Image of Pavonis Mons on the surface of Mars, showing a giant cave (dark) at its summit. Pavonis Mons is a giant shield volcano mostly made of stacks of overlapping lava flows. Here, these stacks have collapsed into a giant cave below, forming a steep-sided conical pit. Material has drained into the hole and formed a debris pile around 62 metres tall in the middle of the pit. Estimates suggest that the cave itself was originally around 90 metres deep before the collapse and infilling. Imaged by the HiRISE instrument on NASA's Mars Reconnaissance Orbiter (MRO).