Okay, secret levels.
Keep in mind this is
all purely from memory - I could well be missing some details here.
Secret level 1: Accessible from levels 3 and 4
Level 3 exit: From the red door facing back into the rest of the mine, there is the yellow door on the left and another room on the right (accessible through a door or energy centre). About midway through this room, there is a secret door in a little cranny to the left-hand side. The secret exit is pretty obvious from here.
Level 4 exit: Above the boss room, there are four secret doors, one for each side; three contain not much but robots - the other contains a secret exit (although it might have been behind a second secret door).
Secret level 2: Accessible from levels 5 and 8
Level 5 exit: Nice and easy. From the start room, go immediately out the door (you're going to need to kill some robots of course). In the first chasm in front of the door, there is a secret door on the right-hand wall about halfway down and in the centre. Behind it lies the secret exit.
Level 8 exit: My memory of this is -very- hazy but I think I know where it is; below the start room is a water-filled room, and off that comes this long tunnel with a robot generator way down at the end. Go all the way down this, slide up, and look left; there should be a door there to a room.
At the top of this room, on the left and right, are two secret doors; they will open, I think, once you cross the room, to reveal a couple of Seekers. However, that's not all; behind these two (from memory) are another couple of secret doors, and one harbours the secret exit (probably on the left).
Again... I am really not sure I have a clue here.
Secret level 3: Accessible from level 12 (most are accessible from two distinct levels, but I don't recall any other for this one)
Level 12 exit: When you're in the yellow door, trying to find the red key, you will run into a couple of tall cylindrical rooms (one through a door in the roof; the second is through a square gap in the wall in the first room). In the second room, there is a grate at the bottom, but there is something down there; hunt around and you will find a couple secret doors in succession hiding this little metal tile room with a secret exit at the end. It's arguably too far from the boss room to be any use for anything but ammunition and refueling - not to mention, thanks to a thiefbot presence, you need to watch your back all the time as well - but it's there if you want it. (I barely remember secret level 3 at all, though. I have played it and nabbed a few secrets there, but it's probably the one I've touched the least in the entire Descent series...)
Secret level 4: Accessible from level 14 (I for some reason thought there was a level 16 exit, but since I can't remember it and TB didn't list it, there probably is none)
Level 14 exit: Again a double-secret-door trick; not very well protected but deviously hidden.
I have great trouble placing exactly where this one was, too. It was not far outside the red door; there was a room between that pastel green room and the secret exit. I think the room was reached from on top of that structure in the green room, since the one on the bottom is probably just a tunnel leading toward the exit.
Anyway, quite close to the entrance to this second room - a violet streaky blue I think - there is another door going out. There should be a tall but short space here; somewhere in the metal-tiled floor there is a secret door.
Inside this secret door is another one, though - and this door reveals the secret exit.
(Sorry if this guide is excessively vague but it's been a while since I've played that level, and I don't tend to memorise them very well...)
Secret level 5: Accessible from levels 17 and 20
Level 17 exit: On the way to the ordinary exit - just as you get into the room where you can find it - there is a shaft going down straight in front of you. This shaft ends in a room - and somewhere off the side is a room with lava in a little recession in the floor. (This room may be behind a secret door - look around.)
There is a bridge over the lava, from what I recall. On one side of this bridge, on the wall in that direction, there is a secret door that hides the secret exit (not sure whether it was left or right).
Level 20 exit: Considering this one is in the boss room, it's a real pain to get to, but I remember it fairly well.
Once you get to the centre floor of the boss room - the one that holds the cloak and shields behind a grate - go to the cut in the floor nearest to the red doors. Turn around and face toward the cloak; you should see a trigger on the pillar on one side of the cut. Blow this trigger, and, I believe, a wall will open ahead of you and to the right; the secret exit should be easily apparent behind it.
Very, VERY nice secret level by the way. And note - when you return from the secret level into level 20 you will be placed somewhere safe, not inside the boss room itself. Just prepare for a little journey back to the boss room afterward.
Secret level 6: Accessible from levels 21 and 24
Level 21 exit: Probably involes blasting a switch to open a wall ... but, truth be told, since I actually am not sure which of the two levels that come to mind was even level 21, I have no idea exactly where this one is.
Level 24 exit: You're more likely to need it here anyway.
The deal with this one is as follows; you're going to find a fairly big room with a bunch of switches at the bottom once you go through the blue door. One of these switches, if memory serves, opens a secret door at the top of the room on the side (it's the one to the right of the blue door, or it is to the right when you come into the room). Afterburn up to this and shoot the trigger on the reverse; it will open a door on the other side which leads to the secret exit.
Actually, there may be an extra pair of secret doors covering these two, but you will certainly find locked doors on the left and right sides of the room; these are the ones that are opened.
Be quick - when you blow the triggers, the doors are immediately opened and will eventually close; if this happens you will not be able to get back through again.
