Reach the Mega Sphere in Doom 64 level 22, Burnt Offerings:

When you first appear on this map, you'll start in a small
cave.  Exit the cave and go right, through 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.