If humans are to truly become interplanetary settlers, we're going to need to have access to water — a lot of it. But loading it on a rocket would be heavy, and trying to escape Earth's gravity with all that weight would be costly. Satellite observations by orbiters around Mars suggest there is an ice sheet in a flat plain called Arcadia Planitia at roughly 35 degrees north latitude. It's a site that both NASA and SpaceX are considering for future human exploration. In this new study, researchers found that it's not just an ice sheet, but a kind of shallow river of ice, similar to what we see in regions of Antarctica.