Jumping the gap in level 9, Nessus:

A short, ground-level hallway runs horizontally along the
middle of the north edge of the map.  Another, vertical
hallway crosses OVER the horizontal hallway, and so forms a
plus-shaped intersection.  The south end of this
intersection runs through a false, intangible wall, and
leads into the large building in the middle of the map.
From inside this building, you can run north through the
false wall, jumping the gap (which is the east-west
hallway) and land in the north end of the intersection,
which is a short, dead-end hallway or cul-de-sac.

In Deathmatch games, this jump can be accomplished simply
by running forward/north from the main building, over the
gap, and into the northern cul-de-sac.

To accomplish this jump in single-player games, you may
need to run BACKWARDS/north through the false wall in order
to make the jump.
This jump can also be accomplished by strafe-running (using
a simultaneous combination of Run plus Strafe, which yields
a speed slightly higher than plain running, but moves the
player character at an odd angle).

Note that with either technique it's helpful to have a long
running start in order to build up speed for the jump.