Entering the blue secret exit box in SNES Doom level E3M5,
Mt. Erebus (how to get to level E3M8, Warrens):

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, which will transport the player to E3M8: Warrens.

To get to the blue box, first check your Automap... Near
the middle of the map is a large, rectangular, diagonally
oriented building.  Its recessed entrance, which is flanked
on either side by a pair of small windows, is on the
southeast face of the building.

Enter this building, and look at the wall to your left.
Down on the floor near the middle of this wall is a
"candle".  The wall panel just 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 ledge on the west side of a
building located just to the south of the blue box.  You'll
be facing south.  If you turn to face northeast, you'll see
the blue box ahead and below you, in the water.

From here you can simply Run (B button in SNES Doom) to
jump into the open top of the box.

If for any reason you miss the box and fall into the water,
you can look around towards the south side of the building
you were standing on for a teleporter.  This will take you
back to the entrance of the large rectangular building near
the middle of the map.