Hrad Vallis is a ~370 m deep, ~800 km long depression located at 34¿N, 218¿W, in Elysium Planitia, Mars. A distinctive deposit, present on both sides of the source area of Hrad Vallis, has a maximum width of ~70 km along the strike of the depression, a lower unit extends for nearly 100 km away from the depression on either side, and an upper unit extends for ~50 km. Viking and Mars Orbiter Camera (MOC) images show lobate deposit boundaries, rheomorphic deformation, and numerous craters 270--580 m in diameter within the deposit. The deposit coincides with a broad topographic rise ~30 km wide and ~100--150 m high extending along the sides of the valley for at least 100 km along strike. We propose that these features formed when a dike underlying the axis of the valley generated a shallow sill, which in turn is partly responsible for the rise topography. The remainder of the rise consists of a mud-like deposit of ejecta from a large-scale phreatomagmatic explosion due to violent mechanical and thermal mixing between the central part of the sill and ice-rich rock layers in the upper few hundred meters of the crust in a fuel-coolant-type interaction. The observed range and thickness of ejecta and the depth of the Hrad depression imply explosion steam pressures up to 360 MPa, ejecta speeds of ~400 m s-1, and a sill at least 150 m thick intruded at a depth of ~350--400 m below the original surface.