The current image of Mars as an arid and hostile desert contrasts sharply with the history revealed by its surface. Channels, minerals altered by water, and other geological traces indicate that the red planet was, in its early days, a much wetter and more dynamic world. Reconstructing how this water-rich environment disappeared remains one of the great challenges of planetary science. Although several processes are known that can explain some of this loss, the fate of much of Martian water remains a mystery.