There is a scene where Lt. Cmdr. Hitchcock sneaks up behind Frank Cobb and hits him in the head with a shovel. But the angle at which the shovel is shown swinging suggests the person hitting him was in front of him and where a wall would be.
While Lenny and Cobb are fighting in the tunnels there is a large rock on the floor. They keep bumping into it, and the way it moves indicates that it is made out of foam or something very light, obviously not rock.