Hidden/secret/inaccessible area(s) in PC Plutonia level 30,
The Gateway of Hell:

Plutonia level 30 has no score-affecting secret areas, but
the map is divided into a few different sections, some of
which are only accessible under certain conditions.

In single-player games, the player will begin in a circular
chamber.  This is one of 4 identical chambers that surround
a lower rectangular chamber.
Those 3 extra circular chambers are for other players in
Cooperative-mode games, and so are not normally accessible
in single-player mode.

In addition to the large, main single-player area, there is
a separate Deathmatch-only area that occupies the southeast
quadrant of the level.  Players starting a Deathmatch game
will appear in that section of the level, and will not have
access to the single-player area.

At the north end of the single-player area is a demon-faced
wall.  Behind it is a small hidden chamber that is normally
inaccessible to players.  (Click for spoilers/secrets.)

Just to the south of the long, curving stairway/tunnel on
the east side of the single-player area, there is a plain
rectangular room which is normally inaccessible to players.
This room has a crushing ceiling that is used for disposing
of some of the monsters that are created on this map.

Monsters can be spawned into that room directly, and/or may
also beam into the room through either one of the 2 monster
teleporters in the north end of the single-player area.