This level occupies the map slot MAP18. For other maps which occupy this slot, see Category:MAP18.
After using the teleporter in the blue skull key area, shoot at the lion face in front of you to lower the wall. Cross the two stone bridges (the first one starts rising as you leave the destination teleporter) in front of you. As you arrive in the cave south of the plasma gun, open the canvas on the southern wall to reveal a teleporter. Use it to reach the nearby platform (sector 554) with the soul sphere.
After killing the spider mastermind, push the canvas in the south of the eastern room to open the wall next to the teleporter. Walk through the opening to find a secret place (sector 540) with a blue armor.
In the north of the yellow skull key is a watery area with a skull switch that raises the bars blocking the yellow skull key room. In the southeast of the corpses is a "window" with a hellfire texture behind it. Push the southern beam with blood on it in the window's frame to open a wall in the north of the window. Enter the new passage to find a room (sector 421) with an UAC logo on the wall and a coffin, containing five rockets and a bulk cell.