How to find the secret exit in level 3, Toxin Refinery,
which leads to the Military Base level:

Look on your Automap.  In the middle of the south edge is a
small room where you enter the level.  This will be the
starting point for these instructions.

Walk straight out of this room, and notice that to the
immediate north is a rectangular pit of green slime.  To
get to the secret exit, you must raise a bridge that will
span this pit.  Here's how to do that:

Turn left and walk clockwise around the outside of the pit,
to a door leading northwest.  Go through the door and head
north up a short flight of steps.  At the top of the steps,
take your first left, down some other stairs.

At the bottom of the stairs, walk left around the corner
and look for a red switch, and activate it.  Now head back
up the stairs you just came down, and then back down the
other steps, and back around to the right, and look for a
newly opened doorway between the two windows.

This doorway is at one end of a narrow hall which leads
northwest up some steps to a large rectangular room.
Near one end of this hallway is an invisible "tripwire"
switch on the floor.  As you cross the tripwire, two narrow
ledges, or "windowsills" on the far walls of the room ahead
of you will drop down.  They'll stay down for a couple of
seconds, then rise back up.
Ultimately, you'll want to do the following in one quick
continuous motion:

Back at the bottom of the hallway, Run up into the room
(which will activate the tripwire) and go right.  Continue
counter-clockwise along the outer edge of the room, past
one window-like indentation on the wall to another, smaller
"window" on the next wall (the room's northwest wall).
This is where one of the "windowsill" ledges drops down
when you step across the tripwire in the hallway.  If the
ledge here is down, step up to the "window", and it will
open.  If the ledge isn't down, go back to the entrance to
the hallway and try again.

If you made it to the ledge before it rose up again, you'll
find yourself looking down another short passage that runs
northwest.  Walk up this path, and you'll come to a small
circular chamber with two pillars stretching floor to
ceiling.  Along the ground at the edge of the chamber is a
stream of green slime.  Follow the stream clockwise around
the room and you'll see a portal in the wall (the larger of
two portals in the room) where the stream continues on out
of the room.

Walk into the portal, and quickly follow the stream around
a tight curve to your left.  There you'll find a platform
in the slime, and a switch on the wall.

Activate the switch and this will raise the bridge back in
the slime pit near the start of the level.

Look to the right of the switch for a slightly discolored
wall panel in the corner.

Activate this panel like a switch and it opens, allowing
you to walk east into a chamber.  Walk to the far end of
the chamber, and a ledge drops down momentarily.  Step on
and ride it up.

Go through the wall that will open up in front of you (or
you can open it manually if it didn't open automatically).
Continue south into a small chamber and walk to the far
wall.  You'll be at a lift which drops down.  Go down and
head south a couple of paces and take your first left up
some steps.

Turn right at the top of the steps and follow the left-hand
wall to a wide door.  This is the door that leads back to
the outer edge of the pit near the level's starting point.
Go through the door and walk around the pit till you get to
the bridge.

Walk north along the bridge and the solid wall ahead of you
will open when you get to it, revealing a tunnel.  Walk
north up the tunnel to a chamber.

As you enter the chamber, the far wall drops down (be wary
of ambushes), opening up to reveal a larger chamber.  Walk
forward to the north wall of the chamber, and the northeast
and northwest wall sections open up.

Walk north through one or the other of these new openings,
and look for a small opening to the north, in the middle of
the curved wall.  The opening gives you access to a short
passage running east.  At the end of the passage is a door.

Go through the door and look right for the secret-exit
switch (or look left for a lift disguised as a wall, which
will take you up...)

PC Doom players can click here to access a demo that shows
how to find the secret exit on this level.