An underground shelter having a flat lid divided into two portions substantially flush with the surrounding ground level and each other when closed having—a first stationary lid portion, and a second moving lid portion, at least one reinforcing member supporting each lid portion; a roller means comprising—at least one linear track, at least one upper track roller, each upper track roller rollingly supporting the second lid portion; at least one ramp defined in the first lid portion, and out of line with the track, adapted to elevate a leading edge roller to a higher level allowing the second lid portion to pass over the first; at least one leading edge roller engaging a ramp; a releasable latch means; a lid retainer means for resisting a lifting force; access means for ingress/egress; and at least one ventilation means for equalizing the pressure.