Water and volcanoes on Mars. Illustration of Mars in the distant past, showing volcanic activity in the Tharsis Ridge volcanoes (right), as well as a young Olympus Mons (upper centre). At this time, liquid water still existed on the surface of Mars, forming an ocean (upper left) across much of the low-lying northern hemisphere. The atmosphere was thick enough to support clouds and rain. This period in Mars' history ended some 3 billion to 3.5 billion years ago, after which Mars cooled down, lost much of its atmosphere and water, and became largely dead geologically. Today it is a cold, dusty desert planet.