Ascending the stairs to the west of the first switch you used in this map, a wall opens in front of you, releasing (two pain elementals on medium and hard skill levels,) eight (seven on easy skills) cacodemons and two lost souls. After this box (sector 66) in the center of the map is opened, run down the staircase and jump across the gap to this platform to register this secret. The box contains six boxes of shells, two blur artifactss and a megasphere.
After using the teleporter behind the blue door, turn left to look at the northern wall: there is a section of wall surrounded by metal bars that has large holes broken into it. Shoot this wall to reveal a passageway lined with four boxes of shells leading to a teleporter. This teleporter leads to the shovel-shaped platform (sector 79) in the west, where you can obtain a BFG9000 (and five bulk cells on hard skill levels).
Routes and tricks
This map is one of the few maps in Memento Mori that can be completed in under a minute, and one of the few that can be finished in less than 30 seconds. The western teleporter in the blood pit takes you to the upper half of the stairway, and can be used to skip much of the level. After that, it's a matter of speeding through the rest of the path while weathering the enemies' attacks, with the help of the soul sphere in the blue keycard compartment.