Follow the water is the mantra for NASA’s Mars exploration program. But present-day Mars is so cold, and its atmosphere so thin, that liquid water cannot exist on the planet’s surface. What NASA can look for, though, is evidence that water was present and active on Mars in the distant past. There are strong indications, in images taken by cameras aboard orbiting spacecraft, that features of the Martian landscape have been carved by water. But some scientists argue that these features could have been caused by short-lived torrents of water – flash floods.