Photogeological studies of the Elysium volcanic province, Mars, show that its sinuous channels are part of a large deposit that was probably emplaced as a series of huge lahars. Some flows extend 1000 km from their sources. The deposits are thought to be lahars on the basis of evidence that they were (1) gravity-driven mass-flow deposits (lobate outlines, steep snouts, smooth medial channels, and rough lateral deposits; deposits narrow and widen in accord with topography, and extend downslope); (2) wet (channeled surfaces, draining features); and (3) associated with volcanism (the deposits and channels extend from a system of fractures which also fed lava flows). Heat associated with magmatism probably melted ground ice below the Elysium volcanoes and formed a muddy slurry that issued out of regional fractures and spread over the adjoining plain. The identification of these lahars adds to the evidence that Mars has a substantial volatile-element endowment.