After flipping all four switches at the start of the map, walk through the window in the northeastern corner to enter a dark room. While you find a box of shotgun shells in its southeast, there is a soul sphere in the lit part (sector 203). Note that once you passed through the teleporter before doing this, you are unable to obtain this secret, as you cannot return to this room.
After entering the first teleporter, stand half on the first elevator and half on the stone floor to its west. Push the skull switch to activate the elevator and go to the room's northwest to find that a secret compartment (sector 253) has opened, containing a partial invisibility and two boxes of bullets.
At the first bend after the first elevator, run southwards into the marble room, head up the stairs in its south, drop into the hole and enter the teleporter to its south to be taken to an outside area with a pentagram carved in the floor and a bloodfall. There are three (ten on hard skills) imps, a spectre, a shell box and - behind the bloodfall - a blue skull key. Grab it and carefully pass the blue door in the west as entering this new room (sector 100) releases two cacodemons (on easy) or barons of hell (on medium and hard skills) on the platform next to the BFG9000's plinth. Ascend the northern stairs to get the BFG first. On the platform, crossing the line between the southern bloodriver and the floor to its south opens two alcoves beside the blue door and one in the south of the room, releasing (skill dependent) two/three/none cacodemons or none/none/three revenants. The southeastern alcove contains two armor bonuses(, the northeastern one a medikit on hard skills) and the southern one the door to the next room, from where none/one/three revenants may approach you. Enter it after dealing with the ambush to encounter a deaf hell knight and (not on easy) an also deaf arachnotron. There are also two armor bonuses. By the way secret #6 is accessed here too.
In the marble room mentioned in secret #3 is a small bloodfall in the southwestern corner. Do a running jump from the topmost step (with the candles and the blood puddle on it, not the one with the chaingun) of the stairway to this bloodfall and open the satyr head to the west to find a blue armor (sector 282). Note that jumping there is rather tricky and may take a few tries; this is easier done by straferunning.
In the southernmost wooden corridor west of the blood pool mentioned in secret #7, the westernmost portion of the southern wall has a plank across the top. Open it to find another corridor (sector 132) with a berserk pack.
The room accessible from the stairway in the blood pool (or from secret #3) contains two switches. The northeastern one lowers a chainsaw at the east with a closed eye behind it. Shoot the eye and a secret alcove (sector 168) will be revealed, containing a box of rockets and a computer map.
In the same room, flip the southeastern switch and run to the blood pool in the southwest as fast as you can. A compartment (sector 162) will have opened, revealing a plasma gun; grab it before the compartment closes. It can also be opened from inside.