A Peculiar Hole on Mars

This hole on the surface of Mars, imaged by the Mars Reconnaissance Orbiter appears to be an opening to an underground cavern roughly 20 meters deep. Why there is a circular crater surrounding this hole remains a topic of speculation, as is the full extent of the underlying cavern. Holes such as this are of particular interest because their interior caves are relatively protected from the harsh surface of Mars, making them relatively good candidates to contain Martian life.