How to exit PC Final Doom Plutonia level 16, The Omen:

First, pick up the blue key at the south end of the level.
Go through the blue-keyed door located northeast of the
level's starting point, and grab the yellow key.  Go back
to where you got the blue key, and head southeast to find a
yellow-keyed door.
Go through that door and you'll be in a room full of rising
and falling pedestals.
One pedestal doesn't move.  It's near the middle of the
room, and towards the yellow-keyed door you stepped through
a moment ago.  On top of this stationary pedestal is a
teleporter pad.
Step on to one of the moving pedestals that's right next to
the stationary one, and turn towards the teleporter.  When
the pedestal you're on rises as high as it will go, Run to
jump on to the teleporter, and you'll be transported to the
exit area.

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