Map: 15 (Industrial Zone)
Source Port: Crispy Doom 4.3
A true UV-Max demo of map 15! Skip to 3:00 to see the trick, I wouldn't recommend watching the rest as it's slow and mostly just for the purpose of showing 100% kills, items and secrets on the intermission screen.
While the secret location has always been known, it has been considered impossible to trigger it and thus get 100% secrets in the intermission screen, from the doomwiki ( https://doomwiki.org/wiki/MAP15:_Industrial_Zone_(Doom_II) ): "Bugs: It is impossible to register secret #4, as the secret sector in question (sector 147) is located adjacent to the raised teleport pad and is only 16 units wide. This setup completely prevents the player from touching this sector's floor (as is required to trigger a secret), because upon crossing the sector's boundary they are immediately raised onto the pad and teleport away. Therefore, the maximum SECRETS percentage one can get on this map is 90%. "
In other words, a secret can only be triggered if doomguys center is within the secret area and if he is on the same height as the secret sector. The second being impossible because doomguy is always lifted up to the teleporter sector floor height when his center is either exactly on or over the secret sector.
It has been possible to trigger it by using idclip, but this is the first demo to show this secret being triggered and hence no use of any cheats (idclip does not work in demos).
This trick has something to do with the moving door and the pain elemental spawning a lost soul on top of you. This forces you down to the lowest floor the moving door is on, which puts you within the secret sector and on the same height, thus triggering the secret.
Basically this means that it is now possible to get 100% secrets on all levels for Doom 2, without the use of cheats.
So to all the commenters in my Nightmare all secrets video about only getting 90% secrets on map 15, you were right, my bad. And to all those completionists who could never get the 100%, well, looks like you didn't try hard enough.. you should have just dragged an uncooperative pain elemental from his hideout to help you out.
The old Doom II UV-Max record for map 15 in 5m36s can be found here together with the rest of the levels: https://www.youtube.com/watch?v=pRa6f3ULLxE&index=16&list=PLTmV4zpWzC8mi4CAaW2hk-bBuyOkqltU8&t=0s
If you want to see more, I also discovered it's possible to go back into the level if you go out of it (out of bounds / void glide), which would be useful for TAS demos: https://www.youtube.com/watch?v=aXtV82Nv0ms
Q: What about Map 27 secret?
A: There is a secret on a teleporter which is difficult to register because the teleporter is activated first. It's possible to get it by doing a linedef skip on the teleporter line, which was discovered years ago. That is a rather precise trick and not easy to do, you can see it in my Nightmare all secrets video here (at 44m10s): https://www.youtube.com/watch?v=yIxEEgEuhT4&t=2650s
Q: Does this work in MS-DOS?
A: Yes, demo can be found on doomworld, so download that and use some ancient computer to verify it if you want. Dosbox displays 100% secrets when playing the demo.
Q: How do you know someone hasn't done this before?
A: First of all you need to get a pain elemental to this very specific area, then you need to be standing on the teleporter itself which isn't a big area (it's very easy to cross the teleporter linedef being so close to it), then you need the door to be moving, then you need the pain elemental to be attacking while he is close enough. If the pain elemental isn't close enough he won't spawn the lost soul on top of you and the lost soul will most likely make you activate the teleporter before registering the secret. Those things simply do not happen by accident and there is no reason to suspect it should even work. Some modern sourceports makes it possible to register, cheats works too, but this works in dos/vanilla without any cheats.
I've seen about 50 people claim to have done this by now, not a single one describes the use of any pain elemental or why they would even try it in the first place. Again, this does not work with other enemies or by using rocket splash etc.
Q: I don't see any secret?
A: The point is that it was previously impossible to get 100% secrets because it fails to register as explained above. The secret is registered at 3:06, the teleporter is used at 3:55 (which has always worked).
Q: Why does the game look so good/bad?
A: The video is made playing back a demo in a sourceport called prboom+ (and no, again, a sourceport is not the reason why this works here)