Access the Mega Sphere in Doom 64 map 22, BURNT OFFERINGS:

When you first appear on this map, you'll start in a small
cave.  Exit the cave and go left, and you'll see a large
pit.  Step on to the narrow ledge that runs along the outer
edge of the pit, and follow it around to a line of raised
pedestals.  These form a pathway to a large round platform
at the center of the pit.
Go to the pillar at the center of the platform and Activate
a switch you'll find on the far side of the pillar.
(If you fall into the water at the bottom of the pit, look
for a raised pedestal against the side of the pit opposite
the switch on the platform.  Activate this pedestal like a
switch, and it drops down momentarily, allowing you to ride
back up to the edge of the pit.)
Now go back to the entrance of the cave where you first
appeared on this map.
Next to the cave entrance is an opening that leads to a
large open courtyard.  Go into the courtyard and head to
the opposite side.
You'll enter a large, dark rectangular room, where an array
of square pillars stretch from floor to ceiling.  Watch out
for Spectres lurking here.
As you first enter this large dark area, turn left, and
look along the left-hand wall for a door.  Open it and step
through.  You'll be at the top of a stairway.  Go down and
take a right through the first open doorway.
Go up the stairs and you'll see a blue-keyed door on your
left.  Just in front of you (to the right of the blue-keyed
door), search the wall for a secret panel.  It will open to
reveal a very small room with a teleporter.  Step on the
teleporter, and you'll materialize next to the Mega Sphere.

Hope this helps. :)                                                

--Led

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