This map's entry in the map archive: The Faults Within by mfx (ad_e3m2.bsp from ad_e3m2.zip)

To get the grenade launcher which is automatically hidden by a moving demon-faced wall in the first room, jump onto the platform where the grenade launcher was and turn around. Look up to see a shootable button. Shoot it to make the grenade launcher accessible again.

Reward: Grenade Launcher

In the area above the lava pit where you have to jump down and press the button that then disappears into the lava, there is a shootable button to the left of the button you have to press to gain access to the button that subsequently disappears into the lava. Shoot it to open up some stairs leading to yellow armour.

Reward: Yellow Armour

To the left of the silver key, there is a shootable button underneath the air duct. Shoot it to open the entrance to the yellow armour that you can grab once you travel up on the silver key lift.

Reward: Yellow Armour

Once you've grabbed the armour (see secret 3 above), drop down to the ducts underneath to find a lightning gun located almost directly underneath where the armour was.

Reward: Lightning Gun

As you enter the final arena, to your left there is an alcove with a nail piercer. Jump onto the ledges to get to this alcove.

Reward: Nail Piercer