A Hole in Mars

What created this unusual hole in Mars? The hole was discovered by chance on images of the dusty slopes of
Mars' Pavonis Mons volcano taken by the HiRISE instrument aboard the robotic Mars Reconnaissance Orbiter
currently circling Mars. The hole appears to be an opening to an underground cavern, partly illuminated on
the image right. Analysis of this and follow-up images revealed the opening to be about 35 meters across,
while the interior shadow angle indicates that the underlying cavern is 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.