Mareotis Tholus, Mars

Rapid eruptions of fluid lava covered much of the northern lowlands with smooth volcanic deposits. Some of these plains have many small volcanoes like this one, Mareotis Tholus. This remarkably detailed image of its summit shows an elongate feature, probably a volcanic vent, and a circular impact crater. Individual blocks that fell from the rim and rolled down through the dusty terrain into the vent are clearly visible.

Mars Reconnaissance Orbiter, HiRISE