Someday soon, robots will venture into the sunless depths of lunar craters to find ancient water ice. New research indicates that some lunar craters also contain double-shadowed regions, or "shadows within shadows," said Patrick O'Brien, a graduate student at the University of Arizona, who presented evidence for the idea in Houston. While permanently shadowed regions don’t experience direct sunlight, most receive some reflected light bouncing off the crater's rim, and this can melt ice.