Bill Webb (tsathogga) of Frog God Games recently announced that his company will release an expanded version of his Rappan Athuk megadungeon "next summer as a hardbound, library-stitched book in both Pathfinder and Swords and Wizardry formats".
He further shared some of the inspiration for the origins of this megadungeon:
"The background of the big announcement goes all the way back to 1978, when I read a small book by Gary Gygax detailing the use of outdoor and wilderness adventures in D&D. This book, along with what has affectionately been termed the “Skull Dungeon” of John [Eric] Holmes fame, formed the basis of my thinking when I began to write my own vast dungeon that I called Rappan Athuk". The "small book" is Vol 3 of OD&D (The Underworld & Wilderness Adventures), and the "Skull Dungeon" is the "Sample Cross Section of Levels" from page 39 of the Blue Book: