A manhole cover assembly configured to behave as a relief valve in an explosion event, opening so that fluid, such as air or explosive gas, may vent from an underground area. The manhole cover assembly may include a manhole cover and a frame, and may be configured to limit movement of the manhole cover with respect to the frame in response to increased pressure on the manhole cover. In one embodiment, the manhole cover may include legs having angled surfaces adapted to substantially center or guide the manhole cover in the frame while the manhole cover displaces to release pressure. In another embodiment, one or more legs may be at least one of pivotable with respect to the manhole cover and slidable within the manhole cover to enable configuring the manhole cover between a restraint mode and a removal mode.