A secret from 1994's Doom II, hidden for so long that its trigger was assumed to be a bug, has finally been discovered.
Via RPS it’s on Map 15, and involved a teleporter that would display as being a secret, but would never actually trigger. That’s because it couldn’t be set off by the player simply standing on it: a workaround had to be discovered.
One that Zero Master found was...letting an enemy knock you onto it. Here’s co-creator John Romero with the congratulations (and details):
Saying he just needed to be knocked onto it is actually under-selling it a little. To activate the teleporter, Zero Master has to lure a Pain Elemental into giving him a nudge after a lengthy and very close-range game of cat-and-mouse. You can see the pursuit in this clip (from around 1:45 onwards).