Noah would have loved this. Mars probably had its own colossal flood millions of years ago, when an ice-covered lake cracked open and gushed to the surface. The scenario hints that buried lakes sheltered microbes that may even now lie dormantMovie Camera in subsurface ice.

On Mars, several huge channelsMovie Camera seem to originate in the boulder-strewn floors of deep chasms and impact craters.

"Huge amounts of water had to flow through these channels," says Victor Baker at the University of Arizona in Tucson. But the terrain dates from a time when Mars was evolving into the cold desert we see today. Where did the water come from?

Manuel Roda at Utrecht University in the Netherlands took a closer look at a crater called Aram Chaos. It has a channel 10 kilometres wide and 2 kilometres deep leading away from it. To carve such a channel, Roda and his team calculate that almost 90,000 cubic kilometres of water must have flowed through it for perhaps a month.

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