NASA landed the Curiosity rover in Mars' Gale Crater precisely because it promised to be a sort of open book that revealed a lot about the planet's history. Well, scientists have been busy reading... and it looks like the crater had an intriguing past. Two newly published research papers indicate that Gale was a warm lake for for a period of about 700 million years, ending about 3.1 billion years ago. In other words, it was habitable for a very long time. Even then, groundwater persisted for a long time afterwards.