Mars may once have been a very wet place. A host of clues remain from an earlier era, billions of years ago, hinting that the Red Planet was host to great rivers, lakes and perhaps even an ocean. But some of the clues are contradictory — they don’t all fit together in a coherent whole. Little wonder, then, that the fate of water on Mars is such a hotly debated topic. The reason for the intense interest in Martian water is simple: Without water, there can be no life as we know it. If it has been 3.5 billion years since liquid water was present on Mars, the chance of finding life there is remote. But if water is present on Mars now, however well hidden, life may be holding on in some protected niche.