To find the "official" secrets on level 24, Hell Beneath:

You first begin this level in a room the north edge of the
map.  You'll be facing south towards a rectangular pool of
green slime, a few paces ahead of you through the doorway.
This pool is bordered on 3 sides by a raised walkway.

Look for the section of the walkway that's in the southwest
corner of the slime pool.  Jump down into the pool and
Activate that section of the walkway from below.  It will
drop down into the slime momentarily, revealing a small
3-sided chamber, where you'll find a rocket launcher and
some ammo.
Step into the chamber, and search the south wall to your
left for a secret panel.  When you Activate this panel like
a switch, it rises up momentarily, revealing another small
room to the south.

Secret 1: Stepping into this small room counts as a Secret
discovered.

There are 2 side-by-side teleporter pads here.  The left
one teleports you to the small raised platform above the
northeast corner of the slime pool; the right one teleports
you to the similar platform above the northwest corner of
the slime pool.

To get to the second Secret, you'll first need the red key,
which is found in a back room at the south edge of the west
half of the map.

Once you have the red key, use it to enter the large room
at the east side of the map.  The blue key is found in
this room, in a small vertical niche in the east wall.

Secret 2: In the northeast corner of this room is a torch.
Stand in front of this torch, facing it.  Behind you now
will be a large brown rectangular metal box, which can be
lowered into the floor to reveal a secret area.  (In order
to get credit for discovering the secret, it is recommended
that you quickly walk across the top edges of the box as it
descends.)  Activate that torch like a switch, and the box
will drop into the floor.

Walking over this newly revealed floorspace will cause
walls to rise up around the outer edge of the room (and may
also spring an attack by teleporting monsters).
Once this "wall-trap" is raised, lower it again by using
the switch you'll find on its interior, or, if you're
outside of the room when these walls rise up, Activate the
switch at the east end of the hallway located at the south
edge of the slime pool.

There are no other "official" Secrets on this map.  However
there is a hidden doorway near where the red key is found:
As you pick up the red key, turn to face west, and walk to
the far side of the room.  Turn right and search the north
wall for a secret panel.  When Activated like a switch, it
rises up, allowing you to step out on to the walkway at the
southwest corner of the map.