To cross the huge gap in PC Plutonia map 2, Well of Souls:

The trick to the pit: There is a bridge across it already,
but it's invisible.
When you first enter the pit area, and you're standing at
its south edge, turn left and you'll see a small window in
the rock wall nearby.
Select your pistol and fire a shot through the window at
the control panel inside.  This causes some "guide posts"
to rise up on the north and south edges of the pit.
Walk a straight line between those posts to cross the
invisible bridge.

PC Plutonia players can click here to access a demo that
shows how/where to cross that bridge.