In the same hall where you found secret #1, run into the eastern light on the southern side to lower the northwestern and southeastern elevators. Take the northwestern elevator to find a room (sector 221) with a medikit. The northern part of the eastern wall is a fake wall, pass it to enter a hallway with a set of shells. The western part of the southern wall is also passable, behind it you find a hallway with a chaingunner and a medikit.
Ride up the southwestern lift in the great hall to enter a hallway to the south. Upon entering the third trapezoid light behind the corner to the east, you will be lowered next to a room (sector 165) containing four (two on easy skills) spectres, (a hell knight on hard,) two medikits, a box of shells, a box of rockets and a box of bullets.
From secret #3 proceed the hallway to the east. At the next light, press on the lantern to open a compartment (sector 137) containing a backpack (and a shell box on Ultra-Violence or higher).
After leaving the first long hallway, approach the soul sphere in the southeastern corner; you will be lowered next to a large room (sector 147) with three (two on easy) imps (, two hell knights on hard skills) and eleven health bonuses. There are two switches in the northwestern corner. Push the lion switch (the eastern one) to lower the lift and exit the room. The satyr switch (the western one) will raise a staircase in the passageway into the water. However, doing this will prevent you from obtaining secret #9.
Exit secret #5 and proceed westwards, then southwards. After lowering the elevator in the eastern wall to access the room with the rising staircase, stay on the elevator until it rises; you will find a roughly L-shaped passage (sector 195) with one (two on hard) chaingunner and a switch. Flip the switch to reveal a teleporter leading to a secret room with two medikits, a stimpack, a backpack and a stockpile of ammo, including (skill dependent) two, three or four bulk cells, a box of shells, a box of bullets, two boxes of rockets, a rocket launcher and a plasma gun.
On the western walkway in the blue key room, bearing the Icon of Sin's face, a portion of the wall has a collection of candles in front of it. Open this wall to reveal a room (sector 407) containing a bulk cell, two medikits, a green armor and a teleporter back into the lava cavern.
After returning from the lava cavern, enter the passage in the southwest of the elevator mentioned in secret #6. Drop down the edge next to the health bonuses and follow the passageway around the corner to the north. Just before entering the water, open the misaligned eastern wall between the candles to reveal a small room (sector 196) containing a teleporter back to the area upstairs, as well as a box of bullets, a backpack and a green armor.
Exit secret #8 and walk towards the water. The sector right before the water is marked as a secret (sector 175). However, if you raised the stairway with the satyr switch in secret #5, this secret will never register due to being too narrow for you to walk on without climbing up a step in the process.
The compartment (sector 167) from which enemies are released in the final, southwestern hallway counts as a secret. This actually consists of two sectors, including the door sector, both marked as secret.
See secret #10 (sector 168).
After passing the door from the map start and walking the hallway to the east, while proceeding to the north you see two candles on the eastern wall. Open the wall between those to find a long room with switch behind a blue armor. Pushing it raises a walkway in the arachnotron's pit outside so you can access the platform in its center, where you find some chaingunners' ammo, a chaingun, a rocket launcher, a bullet box, a set of shells, a rocket box and a medikit.
On the northern wall of the same walkway where the candles are you find two more candles. Open the wall and ride the lift up to access a passage with four imps, three medikits, a bullet box, a partial invisibility and a shell box.
In the great hall, where secrets #1 and #2 are found, enter the western light on the southern wall (where ten health bonuses rest) to lower the floor to a room with three sergeants, four energy cells and two medikits.
Routes and tricks
For years, it was considered impossible to obtain 100% Secrets on this level due to secret #9 being impossible to register, with 90% believed to be the maximum percentage possible. However, in his April 14, 2000 review of Memento Mori , Andy Olivera stated in his description of this level that "contrary to popular belief, you can get 100% Secrets." Eventually, on March 14, 2002, Anthony Soto utilized information from both Olivera and Adam Hegyi to perform his record-holding UV max run of this level, in which 100% Secrets is finally obtained in a demo recording. As indicated above, to obtain secret #9, you must not trigger the stair-raising switch in secret #5, or the secret will not register due to being too narrow for you to step on without climbing up one of the stair steps.