[9-5] E1M4: The Grisly Grotto

From the start, go through the door and up the spiral stairs. Shoot the red symbols on the left and right at the top to make the ceiling fall, revealing a yellow suit of armor. The knight below may be crushed by the falling ceiling.

From the bottom of the stairs, proceed through the passage until you reach a deep lake. Collect the biosuit, then dive into the water. Directly to your right, you should find a small ledge where you can surface to get a box of rockets.

From the pier where you found the silver key, jump off and swim towards the pier. Surface on the other side. Go through the silver key door and turn left. Follow the passage until you reach a small body of blue slime with a bridge over it. Go to the end of the bridge and ride the platform up. Step on all the touchplates. Two doors will open in the walls. Enter both and press the buttons. The game will announce that a secret cave has opened. Jump onto the bridge and head back out to the deep pool where you obtained the silver key. Along the right wall of the cave, there will be a large, bright circular opening. Enter it and swim up. Inside a small room, you will find a grenade launcher, health, and a slipgate (teleporter) to the secret level.