Open the north wall of the right trap in the start room for blue armor and health potions.
Lower the south elevator in the final blood pool to open a passage in the southeast corner of the blood canal. You will find plenty of cells and a plasma rifle.
After getting the yellow key, shoot all three faces in the chamber with the rocket launcher by the yellow door to open a passage at the northeast corner of the blood canal conainting plenty of rockets.
Enter #3 again from the blood canal and you will find a new room with a BFG.