A large landslide deposit on the south wall of Gangis Chasma contains at least 100 billion m3 of material that moved 60 km across the trough floor at a speed of more than 100 km/hr. The deposit consists of slump blocks at the head, hummocky material farther out, and a vast apron of longitudinally ridged material extending to the toe. The landslide deposit resembles many terrestrial ones but is much larger, and differs from most in having longitudinal rather than transverse ridges on its surface. However, some terrestrial landslides also have longitudinal ridges, particularly those in Alaska that traveled long distances over glacial ice and thus were highly efficient. In order to explain this high efficiency, two possible mechanisms of emplacement are singled out. The first involves a sliding motion on a cushion of air; on Mars, this implies a once much denser atmosphere, or the release of a gas, possibly steam. The second mechanism involves a flow motion of debris that is lubricated by some water; on Mars, the water may have come from ice in the source rock. An analysis of the possible causes for the Martian slide shows that the Martian trough walls are highly susceptible to sliding, and that the landslide may have been triggered by a Mars quake.