mrcgran writes "Space.com is reporting a very deep hole found on Mars: 'The geological oddity measures some 330 feet (100 meters) across and is located on an otherwise bright dusty lava plain to the northeast of Arsia Mons, one of the four giant Tharsis volcanoes on the red planet. The Mars Reconnaissance Orbiter (MRO) used its High Resolution Imaging Science Experiment (HiRISE) instrument to draw a bead on the apparent deep hole — a feature that may cause more scientists to ponder about potential subsurface biology on Mars. Because the spot lacks a raised rim or tossed out material called ejecta, researchers have ruled out the pit being an impact crater. No walls or other details can be seen inside the hole, and so any possible walls might be perfectly vertical and extremely dark or — more likely — overhanging.' The original image and its cutout at full resolution can be found in the HiRISE site."