The Six Depths of Terrorism is the fourth mod made by Barry Christian. It was released on October 10, 1998.
It was one of the earliest mods to alter the source code, here allowing 60 seamless levels to play.
Levels 9 and onward would not show end level stats. This, along with a few mapping bugs (including one where the elevator at level 49 throws you back to level 10) were fixed in the SDL port.
Level design is mainly large and spacious, and a labyrinth part in the middle of the mod makes for quite a challenge. All in all a pretty good job for 1998.
An SDL port was released on July 13, 2013.