Layered terrains on Mars discovered just last year by NASA’s Mars Global Surveyor spacecraft bear a striking resemblance to sedimentary deposits here on Earth that form under water. Liquid water is scarce on Mars nowadays, but it might have been common four billion years ago. If these Martian layers turn out to have a watery origin, as some scientists suspect, they could hold the key to the mysterious history of water (and maybe even life) on the Red Planet. Studying Mars’s sedimentary deposits, if that’s what they are, could tell scientists if water existed on Mars long enough for primitive life to begin. Indeed, the layered rocks themselves may be the best places to go fossil hunting.