How to find the Secrets in Doom level E4M1, Hell Beneath:

(Note to Old/Original-Xbox Ultimate Doom players: There are
2 score-affecting secret areas on this map in all versions
of the game that include this map.  However, in Old-Xbox
Ultimate Doom the secret sector in the eastern-most room is
inaccessible, due to the removal of a switch when this map
was ported from the original PC game.  This map edit also
means that 4 Baron Of Hell monsters will remain trapped and
unkillable.)

You first begin this level in a room on the north edge of
the map.  You'll be facing south towards a rectangular pool
of green slime, located 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 niche in the east wall.

In the northeast corner of this room is a torch.  Walk up
to it, but don't touch it yet.  Activating this torch will
cause the large, brown, rectangular block behind you to
sink into the floor, revealing a "NIN" logo.  The easiest
way to get credit for discovering that Secret is to step
onto that block before it sinks all the way into the floor.
Otherwise, the raised "NIN" letters that remain will tend
to get in the way, making it difficult to touch the top of
the block... and if you don't make contact, you don't get
credit.

Secret 2: Activate that torch and step on the block before
it disappears into the floor.  (If the block is already
lowered, then try stepping on the triangular floor spaces
within the "N" characters of the design.)

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.

PC Doom players can click here to access a demo that shows
one way to find all official Secrets on this level.