Caverns of Darkness

Caverns of Darkness is a 12-level mapset for Doom 2 by The Chaos Crew in 2002, using a custom source port, based on the Eternity Engine v3.29 called the "COD Engine" whose source code was released on May 4, 2002 by Christopher Lutz. Despite this, the source code was long considered lost, until it was re-released on May 26, 2019 by Joel Murdoch. Because of the custom engine, or a DOS emulator such as  is required to play. The modification is well known for its liberal use of fake three-dimensional constructs using middle textures, and ambient sounds.

ZDoom patch
In 2008, Graf Zahl released a patch for Caverns of Darkness, allowing it to be played using ZDoom. It works by substituting DECORATE actors in place of the original hard-coded custom thing types and weapons, as well as using MAPINFO to use a custom map translator to account for the COD engine's new sector and linedef types.

Levels

 * MAP01: Surface by Christopher Lutz
 * MAP02: Six Feet Under by Christopher Lutz (exit to MAP11)
 * MAP03: Hell & High Water by Christopher Lutz
 * MAP04: Hydrosfear by NokturnuS
 * MAP05: Command Control by Emil Brundage (NaturalTvventy) (exit to MAP12)
 * MAP06: Periditions Abyss by Christopher Lutz
 * MAP07: Lava Processing by Christopher Lutz
 * MAP08: The Pit by Christopher Lutz
 * MAP09: Hellmine by NokturnuS
 * MAP10: Caverns of Darkness by Christopher Lutz
 * MAP11: Symbol of Grace by Christopher Lutz (secret map)
 * MAP12: The Stand by Christopher Lutz (secret map)