Earlier this year, NASA’s Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) captured an image of a crater with a block dot in the center on the slopes of the Pavonis Mons volcano. Mission managers decided to have another look, this time with the High Resolution Imaging Science Experiment (HiRISE).
The black dot turned out to be a skylight 35 meters across leading into a cavern below. The cavern is likely a lava tube located beneath the surface of the volcano. Based on the shadow, the floor is about 20 meters below the skylight.
Speculation centers on the formation of the crater. It could be a sink hole formed from the loose dust on the volcano flowing into the cavern when the roof collapsed. In this closeup, one can see how the walls of the crater have slumped and material has flowed into the cavern below.
The hole might also be from a small meteorite that punched a hole through the lava tube. There does not seem to be much (if any) rim from a meteorite strike and the crater it would have created. Then, there is speculation that the underground cavern is a former ice deposit that has been exposed and sublimated into the Martian atmosphere.
Later this year, the HiRISE mission plans another image of the crater, creating a stereo picture that may help resolve the mystery.