To access the rooms at the edge of the huge pit on Doom 64
level 6, Alpha Quadrant:

On this level is a very deep, dark, pit.  On the Automap,
it appears somewhat oval-shaped.  The entrance to the pit
area is at one end of the oval.
Off to one side of pit, at the top edge, are two chambers.
When you first enter the pit area, these chambers are off
ahead of you, and to the right.

Three very tall pillars in a line rise up from the floor of
the pit.  Run (remember your Run button) across the pillars
until you get to the middle one.

Note: If you fall into the pit at any time, you can climb
back out by moving back and forth against the stair-stepped
walls that lead up to the chambers at the edge of the pit.
(You may wish to switch to wireframe-Automap view first.)
When you get to the top, walk into the empty space between
the chambers.  When you walk towards the back wall, a pair
of teleporters will open up to your left and right.  Step
into either one, and you'll be returned to the entry point
of the pit area.

Once you're on the middle pillar, turn to face towards the
two chambers.
Switch to your wireframe-Automap view.  Orient yourself so
that you're facing towards the left-hand chamber.  Back up
a little so you can get a small running start, then Run off
the pillar to jump into that left-hand chamber, through the
center of its window.  This may take a few tries.

Once you're in the left chamber, look for a switch on the
wall.  This switch TEMPORARILY opens a door that's just
next to, and below, the doorway that first let you into the
pit area.  (You may be able to see this door if you step
carefully to the edge of the window, and look towards the
entry point.  Be careful not to step all the way through
the window before you activate the switch.)

Activate the switch, and that door opens.  Quickly, but
carefully, step out of the window, and move along the edge
of the pit until you reach that door, and go through.  If
you don't make it to the door before it closes, or if you
fall into the pit, go back to the twin teleporters and
start over again.

A small passageway leads from this door around and into the
other chamber.  A switch there will unlock a booth in the
room outside (the room you went through just before getting
to the pit).  The booth is near where you'll find the blue
keycard.

Finally, remember to activate the switch at the far end of
the line of pillars (at the end of the room opposite the
entrance doorway) if you haven't already done so.