How to acquire the southwest Soul Sphere in Subterra:

Background: There is a large, dark room in the southwest
quadrant of the Subterra map.  A raised walkway winds
across the corrosive floor of this room, and leads to a
Soul Sphere mounted on a switch/pedestal.

To get the Soul Sphere, first look for a teleport pad down
in the corrosive in the northeast corner of that room.  The
teleporter can send you to one of two destinations:
If you step on the teleporter by crossing its south or west
edge, you'll be beamed back to a point just outside the
entrance to this room.  But, if you step on by crossing its
north or east edge, you'll be beamed directly on top of the
pedestal where the Soul Sphere is.
When you appear there, simply step forward to drop off the
pedestal and land back on the walkway.  Doing so will boost
your Secret score to 100%.

PC Doom players can access a demo of this level which shows
how to get the Soul Sphere, by clicking here.