A method for controllably releasing venting gases from a manhole space around the perimeter of a manhole cover by allowing the manhole cover to rise up in stages upon development of upward forces in an explosion to vent gases developed by the explosion while being restrained from being completely blown free except under extreme conditions. A lock body mounts a latch slide which has an outer end and which can be extended to prevent removal of the manhole cover. When a shear pin fails the lock body pivots down, to a limited extent, and a secondary shear pin can also be included allowing the cover to be blown completely free upon development of pressures of a great magnitude. A slotted skirt is used to direct venting gas flow so as to retard the inflow of free air and attenuate the explosion.