The Phobos monolith is a large, upright boulder on Mars's moon Phobos that was first imaged by the Mars Global Surveyor mission in 1998. It is 279 feet across and towers over the surrounding plain.
The monolith is located close to the crater Stickney, and is believed to be part of the ejecta from an impact sometime in Phobos's past. The small moon is covered with impact scars, and its surface is rich with dust and larger debris that settled after each impact. The monolith stands out in images from the Surveyor mission due to its size and upright orientation.