A very dark spot on Mars could be an entrance to a deep hole or cavern, according to scientists studying imagery taken by NASA's Mars Reconnaissance Orbiter. 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 hole might be the sort of place that could support life or serve as a habitat for future astronauts, researchers speculated. Must be deep The Mars Reconnaissance Orbiter (MRO) used its High Resolution Imaging Science...