We played through level 20 of Marathon 2 awhile ago, God Will Sort the Dead, but we partially missed one prominent feature (the crushers situated above the teleporters) and one that has apparently remained hidden and nearly secret since the game's release. It's been hinted at by the maintainer of the Marathon Story Page:

http://forums.bungie.org/story/?read=...

Now it's been exhausively investigated by PerseusSpartacus:

http://forums.bungie.org/story/?read=...

He opened up the level in a map editor to reveal the location of a hidden invincibility powerup and its trigger-- walking over a powerup some distance away in the level, seemingly unreachable.

This item was apparently not mentioned in the Marathon Spoiler Guide, which lists the switches that trigger the crushers as the only secrets:

http://marathon.bungie.org/spoiler/m2...

Here, playing on Kindergarten so the Pfhor don't get in my way, I demonstrate how the crushers are supposed to work when you trigger them just as the Pfhor spawn in.

Then I use the teleport feature of irons' Co-Op 2.2 script:

http://simplici7y.com/items/co-op

and jump myself across the level in time to get the invincibility pickup before the secret door closes.