Where did the waters of Mars go? Underground. Maybe Martian life did, too.

Mars today is pretty dry. The polar caps have water frozen in them, and there's some buried under the surface at mid-latitudes, but not a whole lot on a planetary scale; maybe enough to cover the entire surface of Mars to a depth of 20 to 40 meters.

Ancient Mars is another story. Billions of years ago there was a lot more. Estimates vary, but it may have been enough to cover the surface of the planet to a depth of 100 meters to as much as 1,500! Clearly, something dried Mars up.

Some of it eva...