Inaccessible hole/tunnel in level 10, Refueling Base:

(Note: For a video demonstration of the walkthrough below,
click here.)

Background: In a room in the southwest corner of the level,
there is a square hole in a wall, near the ceiling.  This
hole is the one open end of a short T-shaped tunnel.
Here's how to get up in there:

Look on your Automap.  Dominating the northeast corner of
the level is a large, somewhat-squarish room, where the
walls are lined with cubbyholes, and each cubbyhole
contains a Zombieman or Shotgun guy.

Go to the very southeast corner of this room, and look for
a cubbyhole which has one, large UAC logo on the back wall.
(This cubbyhole is on the east wall of the room, second one
up from the south.)  Use your fist, chainsaw, pistol,
shotgun or chaingun and attack this wall.  (One punch or
bullet will suffice.)  The wall will rise up, revealing a
short, bracket-shaped ("]"-shaped) passage, that leads
counter-clockwise up and around to the back of the next
cubbyhole up from the south.  Walk to the end of this
passage and you'll see a wall with two, side-by-side Baron
faces.  Walk up to this wall and activate it like a switch.
It opens up.  Now simply walk forward (west) as if to walk
back into the main room.  As you cross the boundary of the
Baron-faced wall, you're teleported to the southwest corner
of the level, and you'll be looking out from behind that
raised-up hole you saw.  From here you can either walk out
and drop down into the room ahead, or look left or right
for some goodies.