[9-5] E1M4: The Grisly Grotto

You'll start by heading through a door, and immediately you'll see a spiral staircase. At the top, on both your left and right, there are these red symbols. Your first move is to shoot them. This is important because it'll cause the ceiling to fall, revealing a yellow suit of armor. Keep an eye out, because sometimes the knight that spawns below can get crushed by the falling ceiling – lucky you if that happens!

After that, head back down to the bottom of the stairs and go through the passage. You'll soon come across a pretty deep lake. Before you jump in, make sure to grab the biosuit nearby – you'll need it for the underwater section. Once you're geared up, dive in. Look to your right; you should spot a small ledge. Surface there, and you'll find a box of rockets waiting for you.

Now, from the pier where you found the silver key, you're going to jump off and swim towards the direction of that pier. Surface on the other side. Find the silver key door, go through it, and then take a left. Follow this passage, and you'll eventually reach a small area filled with blue slime, with a bridge crossing it. Head all the way to the end of this bridge and ride the platform up. Once you're at the top, you'll need to step on all the touchplates you see. Doing this will open up two doors in the walls. Go into both of them and press the buttons inside. The game will then let you know that a secret cave has opened up. Now, head back to the bridge, cross it, and make your way back out to the larger pool where you originally got the silver key. Along the right wall of this cave area, you'll notice a large, bright circular opening. Swim into it and keep going up. Inside this small room, you'll find a grenade launcher, some health, and a slipgate that leads to the secret level.