Of the Levels of the World levels for the original Descent 1, one of the most obtuse is "Titanium Falcon Military Depot" (a.k.a. "T.F. Military Depot"). It starts out in a single room with some enemies. The only door which doesn't need a key (aside from the doors to the player spawn locations and the exit door) is opened by flying over a spot next to the convex corner that's closest to the door in the room.
Behind that door is … another door. It doesn't open when touched/shot. Presumably, it is opened by flying over another trigger spot in the room, but I can't for the life of me find that spot. Does anyone remember where it is?
Frustratingly, I'm pretty sure I figured it out when I first played this level in the 90s. But that memory is gone.
The only mention I can find of this level at all is a ReadMe-ish document, found here, which contains:
Name of Mission: TiTANiUM FALCON MiLiTARY DEPOT Description of mission: This level is a little tricky.....you must hit switches in order to open the doors to get the keys....but once you use your imagination the path will come to you. Use the clues i have provided.......they will help you out. What are behind the doors are either helpful or harmful (most cases both), so be careful!.....find the keys in succession and then advance your ship to the TiTANiUM FALCON to blow the reactor. *Intro as follows (optional):* iNTRODUCTiON: THIS LEVEL IS A MILITARY DEPOT ON SATURN COMPLETE WITH A HANGAR WHICH CONTAINS THE TiTANiUM FALCON, PTMC'S NEWEST TOP SECRET ATTACK CRAFT, WHICH THE MECHS HAD BEEN STATIONED THERE TO GUARD, BUT ONCE THEY WERE INFECTED WITH THE MYSTERIOUS VIRUS, YOU MUST BE SENT IN BEFORE THE TITANIUM FALCON'S SUPERCOMPUTER BECOMES INFECTED....EVERY SECOND THE ViRUS SLOWLY EATS AWAY AT THE CIRCUITS.....UNTIL FINALLY IT WILL BECOME INFECTED AND LAUNCH ITSELF.....PTMC'S HEADQUARTERS IS THE DESTINATION THAT IT WAS TRACKING, AND IF IT OPENS FIRE ON THE HQ, IT WILL CAUSE MASSIVE DESTRUCTION TO THE SPACE STATION. YOU MUST NOT LET THAT HAPPEN........DESTROY THE FALCON'S REACTOR AND VAPORIZE THE WHOLE DEPOT IMMEDIATLEY, BEFORE IT BECOMES INFECTED.
I like to think I'm pretty observant, but I haven't found any "clues" in the main room at all. The trigger zone which opens the first door, I found by accident; now, even knowing where it is, I still can find no clues that would have indicated its location.