This level occupies the map slot MAP24. For other maps which occupy this slot, see Category:MAP24.
At the map start, turn around and walk through the wall behind you. In here (sector 1), you find a shell box, two rocket boxes and a megasphere. Once you left this secret area by riding down one of the four lifts, you cannot return without cheating.
After riding down a lift from secret #1, go to the southeast of the central ring and look to the southeast. Down in the blood pool is a dark cave behind a wall. Straferun towards the wall to jump over it. (If you fail trying so, use the nearby teleporter to get back to the north of the ring.) In here are some corpses, a bulk cell, a soul sphere and an invisibility sphere to collect or save up for later. Use the teleporter (approaching it triggers a sector to lower; you should hear the sound) to get back to the ring's north. From here, take the route east of the ring, through the passage and over the six pillars (each with a health bonus on it). The teleporter gets you up to the ledge in the (south)west of the central ring. Walk to the ledge's southeast and drop down into the hole (if you did not trigger to lower this sector, there is just a niche) between the two metal beams to land on a medikit). Walk on the floor's bright part (sector 92) to register the secret. There are some chaingunners, a shortcut back to the cave south of the ring and a teleporter. Use it to telefrag the arch-vile in the northeast of the central ring. Just walk a step to telefrag the other arch-vile northwest of the ring. Make another step to be teleported again. Descend the stairs and drop into the hole to find an invulnerability sphere. Drop down once more to find yourself again in the dark cave with the corpses southeast of the ring where you return to by using the teleporter again.
After removing the blue bars and entering the now accessible room, go north. You arrive in a hall with a walkway behind a fence to the north and the east. Walk behind the waterfall (sector 437) that fills the well in the southwest of the hall to find a berserk pack.
From secret #3 move on to the west. Eventually you arrive in the map's northwestern main part: a giant cave with a harming blood floor, partially surrounded by walkways. Push the switch north of the entrance to raise some stairs later in the level and follow the walkway to the north, it leads to a rectangular room with a skull switch in the northern wall. Push it to unlock a teleporter. Return to where you entered the giant cave. Run up the steps in the west to jump over the gap and land on a medikit. Follow the bend and look at the teleporter you unlocked before. Jump over right in front of the wall surrounding it to stop your momentum. Before teleporting, look eastwards. There are two holes in the wall, from each coming out a blood fall. The northern (at eye level) one's back wall has a flickering light. Shoot at this back wall to open a niche (sector 358) with a BFG9000 in it in the back of a small cave across the red door. (This cave itself will open when you walk up the stairs to the red skull key.)
After using the unlocked teleporter mentioned in secret #4 and exiting the fenced area to the south but before entering the rock passage, drop down to the west. On the blood floor are some stairs leading down to a gargoyle switch, push it to lower a lift to the switch's south. Quickly ascend the stairs and step on the lift. As it rides up, enter the gap in the southern wall to find a hidden room (sector 298) with a teleporter. It leads to a room with some (two on easy skill levels, four on medium and hard) revenants, a box of shells, four rocket boxes, two backpacks, a soul sphere, a blue armor and a radiation suit. Approaching the blue armor reveals an ambush with four more revenants and another invulnerability sphere. (Instead of the latter there are even four more revenants on hard skill.)