To get to the secret exit on level 15, Industrial Zone (and
access level 31, Wolfenstein):

THE DIAMOND BUILDING
--------------------
First, for a reference point: Look on your Automap.  In the
southern half of the map is a huge lake of crusted lava,
which spans east to west across the map, dipping south in
the middle.  In the center of the lake, against the north
shore, is a huge light-brown building.  On the map it
appears almost diamond-shaped (a diamond with stair-stepped
sides).  This building will be referred to as the diamond
building.
You'll need the yellow key to gain access to the secret
exit, and the yellow key is located within the diamond
building.
You can get into this building through either one of two
ways: The first way is via a teleport pad in the lava lake.
It is a small, red teleport pad on the surface of the lava,
and is nestled in a corner on the west-southwest side of
the diamond building.  This teleporter will beam you into
the heart of the diamond building.  See "THE YELLOW KEY",
below.
The second way into the diamond building requires that you
have the red key.  If you don't have it yet, here's how to
get it:

THE RED KEY
-----------
To the immediate northwest of that central diamond building
is a smaller, squarish building, which is surrounded by a
shallow moat of lava.  Running counter-clockwise up and
around the outside of the building are flights of stairs.
Walk all the way up the stairs to the central platform at
the top, and look for a pair of green torches.  Approach
the green torch on your right (the southern-most torch),
and put your back to it, so that you face west, towards the
center of a tall, brown, column-like structure on the west
edge of the map.
Now Run forward (hold the Shift key while pressing the up
arrow) to Run off the platform you're on and jump across to
that brown column.  You should land on (or near a teleport
pad on a ledge about halfway up the side of the column.
When you step on that teleporter, you'll re-appear on the
ledge of a building located to the north of the brown
column, and you'll be facing east.  A little ahead of you
and to the left will be a long horizontal window.  Walk
forward (east) until the center of the window is to your
immediate left.  Now Run east off this building.  With a
little luck you'll land on the ledge of another building
ahead of you.
Once you're on the ledge, you'll see a lion's-head switch
ahead of you, and a large opening into a room on your left.
Activate the switch, then turn left and drop into the room.
Look for a red teleporter pad on the floor in the southeast
corner of the room, and step on.  When you materialize at
your destination, simply step off the pad, and then step
back on it again.  You'll materialize on top of a stack of
boxes, facing the red key.  Simply walk up and grab it.

TO USE THE RED KEY
------------------
To the north, and a little east, of the diamond building is
another building.  On the western edge of this building is
an exterior elevator, with a face-like switch to the left.
Activate the switch to drop the elevator, then ride it up.
At the top is a door that you can open with the red key.
Walk through the door and follow the path as far south as
it goes.  Turn left (east) and step on the red teleport pad
you'll see there.  You will re-appear on another pad at the
northwestern corner of the diamond building.
When you appear on the pad, you'll see a skull switch off
to your right (south).  Activate the switch and step on the
elevator that will drop down on your right.
Ride it up and follow the ledge you're on until you get to
a large opening on your left (east).  Walk east and drop
down into the room ahead of you.

THE YELLOW KEY
--------------
Ahead of you you'll see a BFG-9000.  Touching it will open
a door ahead of you to the east, and unleash some monsters.
Grab the BFG.
Once you've cleared out the monsters, a panel will open at
the back of the niche to the east as you approach it,
revealing the yellow key.  As you go to pick up the key, a
switch will emerge out of the floor to the west, near the
spot where you entered this room.  Grab the yellow key,
then go activate the switch.  Stairs will appear, leading
up and west, out of the room.

ONCE YOU HAVE THE YELLOW KEY
----------------------------
Walk up the stairs and out, going counter-clockwise around
the ledge, hugging the left-hand wall.  Almost immediately
you'll have to step down to a lower part of the ledge.
When you step down, there will be a golden candle holder
nearby, next to a skull switch that controls an elevator
you just now stepped down from.  Continue counter-clockwise
around the building until you see a second gold candle
holder.  Walk up to it, and turn so that your left side is
touching against this candle holder, and your back is up
against the wall.  You'll be facing south.
Before executing the next maneuver, you may want to expand
your first-person view to fill the screen (no status bar).
You should also be facing as perfectly south as possible.
(You can check this on your Automap, by expanding the map
size with the + key, and making minor adjustments to your
orientation until your white-arrow player icon is perfectly
vertical.)
Now simply walk (not run) south, off the ledge, without
stopping or pausing.  Continue to hold the walk (up-arrow)
key until the instant you land, then release it.  With luck
you'll have landed on a narrow raised platform in the
middle of the southern end of the lava lake.  (If you miss
the platform, turn right and run northwest, looking for a
small, red teleport pad in the lava at the base of the
diamond building, which will be on your right.)
The platform contains some goodies, including a radiation
suit (which you may have automatically picked up as you
landed).  There is also a switch to your left as you land.
Activate that switch, and grab the radiation suit.  The
switch opens a large door on the eastern-most side of the
lava lake.  Jump off the platform you're on and Run to the
east side of the lake and into that opening.  Follow the
tunnel until you find a green Invulnerability Artifact.
Pick it up and Run back out where you came from.
There is a small, red teleport pad on the surface of the
lava, nestled in a corner on the west-southwest side of the
diamond building.  Run to that teleporter and it will beam
you back into the heart of the diamond building.
Run up the stairs to get out of the room you're in.  From
the western ledge you're on, run southeast off the corner
of the ledge.  You should land on the grass, and in the
vicinity of a large yellow-rimmed door located at the base
of a tall tower near the west edge of the map.  Run through
the door and continue south, past a red torch on your
right, and into a low, dark lava tunnel you'll see ahead of
you to the south.  Follow the left-hand wall of this tunnel
and you'll soon come to a new, branching tunnel on your
left.  Follow that branching tunnel, and at the end is the
teleport pad leading to level 31.

To read about or download a demo of accessing the secret
exit which leads to level 31, click here.