Cody Miller (pinkynarftroz on YouTube) demonstrates what we didn't (or rather couldn't... perhaps even wouldn't) do in the original Marathon: use the grenade launcher to climb a sheer wall. The latest version of Aleph One is actually able to duplicate this behavior when working with the original map files now-- the M1A1 scenario that was adapted from the original game for the Marathon Infinity engine can't, which is why some invisible ledges were added to some levels to allow certain places to be reached.