From the map start go west. In the hallway west of the nukage pit, approach the outside area to open two small caves behind you containing enemies, dispose of them. Follow the hallway westwards and go south after descending the stairs. Take the eastern path at the nexus to enter another outdoor area. Push the switch behind the chaingun to raise some stairs that you climb. After passing the door upstairs, you are in front of the southeastern yellow door, follow the sewers northwards to a shadow region (containing a backpack that could be seen from the nukage pit). Stepping into the shadow will connect aforementioned caves to a corridor. Return to this corridor to find a teleporter. Enter it and you will be taken into a sewage tunnel containing a megasphere. Approach the bulk cell in the dead end and the wall will open to the north revealing a teleporter leading back into the nexus. The passageway to the teleporter counts as a secret. (sector 287)
After obtaining the yellow key, ascend the stairs in the west of the nexus. Cross the outdoor area to enter the northwestern computer room; open the yellow door and flip the switch to lower the mainframes. This reveals a blue, circular room (sector 262) with a switch, (two sergeants on medium an hard skills, an arch-vile on hard skills,) a trooper and a stimpack.
Flip the switch in secret #3 and you will be taken up to a compartment (sector 266) containing two (one on easy) chaingunners and a soul sphere. Flip the switch to head back down.
While taking the eastern stairway up in the north of the nexus, press on the eastern wall to find an elevator; take it up to enter a normally inaccessible area (sector 76) containing (skill dependant) none/one/two sergeants, a stimpack and a blur artifact. Flip the southern switch to open the way into the room with the four blue teleports.
In the room with the four blue teleports, open the northern wall (sector 53) right west of the stairs leading down to find a medikit, three sets of shells and a super shotgun.
In the water section that can only be reached with the blue key, open the misaligned part of the northeastern wall (sector 252) to find a health bonus and a teleporter. This leads into the southeastern cage, containing a blue armor and an imp (as well as a chaingunner and a box of shells on Hurt Me Plenty or higher).
Enter the teleporter in secret #7 and open the eastern wall (sector 256) to find another teleporter. This leads to a secret room with a hell knight, two spectres, (skill dependant) none/one/two imps, two medikits, a rocket launcher, five boxes of rockets, a box of bullets and a box of shells. Enter the teleporter in this room to return to the nexus.
During a multiplayer battle, when you are killed, one of the places in which you can respawn is in a short, narrow tunnel (sector 276) in the southwest of the map. Press on either end of the tunnel to open paths into the demons' compartment, and a teleporter that leads to a (multiplayer-only) BFG9000. The eastern door sector (sector 278) counts as a secret. Because of its location, however, it is impossible to obtain this secret in single-player or coop mode unless the player uses the idclipcheat.
In the northwestern computer room mentioned in official secret #3, press on the gray, western terminal from the south, and it will lower to reveal an invulnerability.
Because one secret is placed in a deathmatch-only area, you can only obtain 88% Secrets on this level.