A giant lake of molten rock forms a dark horseshoe on the surface of Io, one of the big moons of Jupiter, in this view from the Galileo spacecraft. Io is the most volcanically active body in the solar system. Loki Patera (just below center) is the largest of its volcanic features — a lake that covers about 7,000 square miles. Fresh lava bubbles up from below to fill its basin. [NASA/JPL]