Mars is cold and dry, but long ago, it was warmer and wetter. Today, its geology is driven by wind and sand, but it was also shaped by water and maybe even glaciers. Glacial activity on Mars was long assumed to be dry, with glaciers frozen right to their beds, scouring the landscape of the Red Planet. But now, researchers think they've found evidence of subglacial melting, where a layer of water forms under the glacier, helping to form various features on Mars.