Today's Image of Mars comes from Mars Odyssey Orbiter's THEMIS and shows Mangala Valles, which was created when Mangala Fossae released a torrent of ground water, carving out the 900km valley we see below.
There is substantial evidence that Mars once had, and may still have underground ice and water. In some underground reservoirs the water cannot break through the rock encasing it, so it has built up over billions of years. When something, like an asteroid impact, ruptures the surface it releases the water onto the surface creating an outflow channel, or valley, like the one below.
Clicking on this image will take you to the original high resolution image from THEMIS. [See their article on Mangala Valles]