Access the secret exit (blue box) on level E3M6, Mt. Erebus
(how to get to level E3M9, Warrens):

(Note: For video demonstrations of the walkthrough below,
click here to watch in first-person mode, or click here to
watch the same demonstration in Automap mode.)

Near the middle of the north edge of the map is an oblong
blue box down in the water.  The box has no top or ceiling,
and from a high-enough vantange point you can see a switch
inside the box, at its north end.  This is the Secret Exit
switch that will transport the player to the Warrens level.

There are three basic ways to access this switch:
1) The original/"official" method, where the player uses a
rocket launcher (and one of the level's Invulnerability
Artifacts) to get inside the box.
2) The Strafe-Run method, which allows the player to leap
into the box from the nearby building.
3) The Edge-Activation trick, where the switch is activated
from outside the blue box.

To get to the blue-box area, first check your Automap...
Near the middle of the map is a large, rectangular,
diagonally oriented building.  Its recessed entrance can be
found on the southeast face of the building, flanked by
small, windowed chambers.

Enter that building, and look at the wall on your left.
Down on the floor near the middle of this wall is a candle.
The wall panel behind the candle is a secret door.  Open it
and step on the teleport pad you'll see there.

You'll be teleported to a platform/ledge on the west side
of a building that is located just south of the blue box.
When you materialize on this ledge you'll be facing south.
If you turn and face northeast, you'll be able to see the
blue box ahead and below, down in the water.

TO ENTER THE BLUE BOX WITH THE ROCKET LAUNCHER
----------------------------------------------
Walk east along the ledge you're on, until the blue box is
directly north of your position.  When you turn to look at
the blue box, the front of your rocket launcher should
appear to be centered in the box, and will visually obscure
the switch inside the box.

Turn to face south (you may want to check your Automap and
zoom in to be sure you're facing directly south), with your
back to the blue box, and the nose of your rocket launcher
up against the wall in front of you.  Save your game here.
You'll be firing the weapon directly into the wall (which
is why an Invulnerability Artifact is recommended).
Note that at Skill levels 2 through 5, the blast from your
rocket launcher will be enough to propel you straight into
the box; But at Skill 1 the explosion is less powerful, so
you will also need to Run backwards while the rocket is
detonating.

If for any reason you miss the box and fall into the water,
simply reload your game, or go around to the south side of
the building you were standing on and look for a teleport
pad.  This will take you back to the entrance of the large
rectangular building near the middle of the map.

TO JUMP (STRAFE-RUN) INTO THE BLUE BOX
--------------------------------------
Once you teleport to the ledge on the building to the south
of the blue box, you can use a simultaneous combination of
Run-forward plus Strafe-sideways to jump into the blue box.
This "strafe-running" manuever provides a somewhat higher
speed than plain Running, but moves the player character at
an odd angle.

It can be difficult to judge the best spot on the ledge to
start strafe-running from, so it's best to kill any nearby
monsters, and then save your game before attempting the
jump.  This way, if you miss the jump you can simply load
your saved game and try again.

One method to find a general starting position to run from:
When you teleport to the ledge, immediately switch to your
Automap, and zoom in (use the plus key in PC/Mac Doom).
Slowly step forward until the very tip of the player arrow
touches the line ahead of you (this line marks one edge of
the ledge you're standing on).
Now turn to face northwest, so that the player arrow is
parallel to the line you just touched.  Slowly walk forward
until the tip of the player arrow again touches the border
line of the ledge, then turn to face directly north.
Switch back to first-person view, and save your game.

To strafe-run on a PC/Mac, with a standard-layout keyboard:
Hold down Shift and Alt simultaneously, and (to strafe-run
forward and right), simultaneously press the Up arrow and
Right arrow.

If for any reason you miss the box and fall into the water,
simply reload your game, or go around to the south side of
the building you were standing on and look for a teleport
pad.  This will take you back to the entrance of the large
rectangular building near the middle of the map.

TO ACTIVATE THE SECRET-EXIT SWITCH FROM OUTSIDE
-----------------------------------------------
On the Automap the blue box is located near the north edge,
and will appear as a rectangle with a somewhat smaller
trapezoid shape inside.  The exit switch is on the interior
north wall of the box.

Outside the box, walk up to one of the long walls (east or
west wall), until you're pushing directly against the wall.
Switch to Automap view, and maneuver so that the player
arrow is just slightly to the south of the switch.
Now pivot to turn the arrow to point toward the center of
the switch (straight through the wall you're against).

The switch can be Activated directly through the east or
west wall, but your positioning must be rather precise.
You may need to press the spacebar/Activate button several
times in a row while making very tiny adjustments to your
position and orientation.  (With some time and practice it
will become easier to find the small activation zone for
the switch.)

PC Doom players can click here to access a demo that shows
all three methods for accessing the secret-exit switch.