Rappan Athuk is a well-known "mega-dungeon" that has a long and complex publication history.
Originally created as a home-campaign dungeon for Dungeons & Dragons (Original Edition), it first was commercially published in 2000-2001 for Dungeons & Dragons (3rd Edition), with some additional "web enhancement" materials made available for download. The materials received a later partial revision and expansion in 2006, for Dungeons & Dragons (3.5 Edition).
In 2012 a runaway Kickstarter campaign resulted in a proliferation of materials for Rappan Athuk to support both Pathfinder Roleplaying Game and Swords & Wizardry. The game items for this are noted in Rappan Athuk PF/SW Kickstarter Rewards, and included Rappan Athuk Original Notes.
In 2018 another Kickstarter funded to revise and further expand the dungeon for Dungeons & Dragons (5th Edition); the new material also is planned to be "back-ported" for Pathfinder Roleplaying Game.
See also Too big to fail...