The Curiosity rover is lucky it got to Mars when it did. If it had arrived 3 billion years ago, it would have had to swim the backstroke.

Last week saw news of modern Mars water running down hills and disappearing. But in Gale Crater, where Curiosity is now poking around, water once pooled into a lake – and then sat there, placid, for hundreds or thousands of years at a time.

Researchers have long suspected that Gale Crater once hosted a lake, but Curiosity’s recent travels have built up the case considerably.

“We had a snippet of the lake, and now we’ve found the mother lode of it,” says team member John Grotzinger of the California Institute of Technology. Depending on the fickle Martian climate, the lake could have grown and shrunk over time, supported for millions of years by a water table underneath.

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