Find the secrets in PSX Doom level 30, Threshold of Pain:

There are 2 "official" secret areas on this level.

Secret 1: When you first appear on this level, you'll start
at the top of a short curving stairway, located on the east
side of the map.
Walk down the stairs to enter the first room of the level,
and then exit this room through the door in the west wall.
Walk to the southwest corner of the next room, and take one
step west, to exit this room.
You'll be standing at the east end of a wide hallway.  If
you turn right, you'll also see a shorter hallway leading
north.  Walk a few paces north, up that hallway.
At the point where this hallway turns left/west, Activate
the north wall like a switch, and it will open, revealing a
long secret chamber.

Secret 2: To access the second secret, you'll need to gain
access to the keys.  (For a walkthrough that tells how to
get both keys on this level, click here.)
As you pick up the blue skull key, step down into the long
hallway that leads east.
Follow the left-hand wall until you come to a new opening
on the left.
Step onto the teleporter you'll find there, and you'll be
beamed to an outdoor structure near the middle of the south
edge of the map.  There is a Megasphere here.