This level occupies the map slot MAP19. For other maps which occupy this slot, see Category:MAP19.
From the map start, drop down into the blood pool and take the eastern passage. Ascend the stairs in the east. Upstairs, climb the stairs in the north of the large room, keep going north, jump down the lift and kill the hell knight. Follow the hallway to the west and to the south. At its end, enter the door to your right. There is a teleporter surrounded by colored lights. Walking into the teleporter over the red lights teleports you back to the map start. Instead, enter the teleporter from the western side that has yellow lights. You end up in a secret room (sector 135). Use the switch to your right to access the aforementioned hallway.
In secret #1, ascend two small steps to find a radiation shielding suit. Stay on this floor and explore the hallway to your south. At its southern end, you find a berserk pack in front of the blood pool in the west of the large room mentioned in secret #1. Instead of dropping down, jump on the balcony' ledge west of the railing and carefully follow it around the corners. Enter the niche to the west of the railing's southern end and open the back wall to find a room with a teleporter in it. Use it to reach a stone platform (sector 742) with six rocket boxes on it in the southeast of a (non-harming) lava floor cave.
After acquiring the yellow skull key, return to the map start. Drop down into the blood pool and take the western passage. Ascend the stairs in the west. Upstairs, enter the room in the north and go through the western door (south of the teleporter). Walk through the yellow door in the south and open the wall between the two large crates to reveal a secret room (sector 315) containing a bulk cell and a blue armor.
Acquiring the red skull key and finishing the map without any other keys: As you step on the lava floor mentioned in official secret#2, one (on easy), three (on medium) or five (on hard skill levels) hell knights teleport in. Do not go too far into the lava cave as there is a deaf arch-vile in the circular room with the torch to the north. Lure the hell knights on the stone platform and kill them here (or even better, lure them into the teleporter and telefrag them) so the arch-vile will not resurrect them. Note that the floor in the northern part of the cave, where you can find a soul sphere, is slightly lower than the one in the southern part and that there is an apparently unreachable walkway high in the rock wall. Look at it's northwestern three segments (sectors 762, 764 and 765) with many corpses and gibs on it. This is the place where you have to get to. From the stone platform with the teleporter on it, walk straight to the cave's western wall to attract the arch-vile's attention and lure him so that he starts attacking you. As he starts charging at you, run from him towards your destination so that you roughly are in a line with him and your target as his attack not only will lift you up vertically but also gives you a horizontal push. His attack must hit you when you still are running on the higher (southern) floor so that you perform an arch-vile jump and land on the target segments. As you land, quickly open the back wall (sector 772) of the middle, lowest segment to reveal a small cave where you can take cover. Kill the arch-vile and use the small cave's teleporter to find a shortcut to the red skull key. Use the southeastern, white teleporter you just left to get back to the map start or the northeastern, red teleporter to get back to the blood pool west of the large room mentioned in official secret #2 (- from there run westwards and ride the lift up to find the red door).