Part of our fascination with cold-and-dry Mars is its warm-and-wet past. What was Mars like when it had liquid water? Did any life swim in it? And where did the water go?

The most obvious explanation for that last question is that Mars’ water slowly diffused into outer space. Because Mars is considerably smaller than the Earth, its interior cooled much more quickly. When the Red Planet’s magnetic field petered out around 4 billion years ago, the loss of this atmosphere-protecting shield would have allowed water vapor to escape.

But that’s not necessarily the whole story. Estimates of Mars’ initial store of water are generally larger than estimates of how much would have escaped to space in this way. What didn’t go up may have gone down—beneath the Martian surface.