Mangala Fossa is a graben in the Memnonia quadrangle of Mars, located near 11.6°S 151.0°W / -11.6; -151.0, which originated in the Hesperian and Amazonian epochs. This dike breached a pressurized aquifer trapped beneath a thick "cryosphere" beneath the surface. As the floor of the graben subsided, water found its way up one or both of the faults in the crust that defined the edges of the graben and spilled into the depression, eventually filling it and overflowing at the lowest point on the rim to erode the Mangala Valles channels. (wikipedia entry)
A fossa is a long, narrow, shallow depression. On Mars they are named after the nearest named albedo feature on the Schiaparelli or Antoniadi maps.