The surface of Mars is known for being an extremely cold, desiccated, and irradiated place. But as its many surface features attest, the red planet was once a warmer, wetter place with flowing water and glaciers. Today, most of the remaining water on the surface is largely confined to its polar regions in the form of ice caps, permafrost, and subsurface glaciers. Nevertheless, the seasonal melting and freezing of this ice still impacts the Martian environment and offers clues about glacial activity in the past.