How to find the blue key in Doom level Mt. Erebus:

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

Near the center of the map is a large, diagonally-oriented
building, with a small wooden door in its southeast wall.
At low skill levels, the blue key will be just outside of
this door.

To get the blue key at higher Skills, open that door, step
in and turn left.  Scan the middle of the southwest wall,
and look for a small candle on the floor at the base of the
wall.  Open the wall panel just behind the candle, and step
on the teleporter that's revealed.

You'll re-appear on top of a small structure located at the
north end of the map, and you'll be facing south.
Nearby and on your right are a couple of small, square
structures to the southwest: A pedestal where a chainsaw
sits, and just to the left of that, a small, square box.

Run to jump into this box, and then step on the teleporter
you'll find inside.  You'll re-appear at the northeast tip
of a 3-spoked building on the west side of the map.

Turn around and go forward and left, to the southeast spoke
of this building, where you can find the blue key.
Go to the west-side spoke and Activate the switch there,
and the tip of the northeast spoke will open up, allowing
you to leave this building.

Click for a map showing the blue key on MT. EREBUS (E3M6).

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