: One possibility is used in a lot of games to simulate drowning while
: swimming: Once you run out of air, your health begins to decrease. This
: gives you a few extra seconds that can make the difference between life
: and death, but still imposes a severe penalty for wasting time.
Actually, there is some sort of link within the game engine. Have you ever run out of O2 while invincible? I don't recall if this is possible in one of the Marathon games as opposed to a 3rd party scenario, but it has definitely happened to me. When this happens, there is a momentary flash/sound similar to when you're hit by weapons fire while invincible, but then nothing happens. You don't suffocate. You can play happily ad infinitum, even after your invincibility runs out, without any oxygen. Theoretically it may be possible to play an entire game this way as long as you don't refill your O2 tanks or grab any oxygen power-ups, but I've never tried it and don't know if you can continue this way after changing levels. It would be an interesting experiment, though.
