I've run the mega-dungeon Rappan Athuk twice so far. Once in AD&D, and currently in 5e. From its marketing, Rappan Athuk is "legendary," but I'm honestly unclear whether that's a claim about real live people knowing and loving the book, or whether it's a reference to fictional people in the game world regarding the fictional dungeon as fictionally legendary. I guess in the end, it doesn't matter. The point is that it's a big, sprawling, messy dungeon, and it's just waiting to be explored.
Warning: This post features spoilers. Not major spoilers, but it's all about entrances to the dungeon, so if you don't want to know anything about Rappan Athuk before playing, abandon this post now.
As far as I can tell, there have been several editions, re-releases, and conversions of Rappan Athuk, to say nothing of expansions and supplements. On the one hand, that sounds amazing because there's so much content! On the other hand, that can be really intimidating because there's so much content. Both times I've run the dungeon, I've run it from a PDF, and I got so thoroughly confused in the AD&D edition that I all but dissected the 5e version of the PDF in preparation for my next game.
First of all, each level of the dungeon got its own PDF. Then I extracted all the maps. (Edit: I recently discovered that you can instead purchase player and DM versions of the maps, which is well worth it when running the game virtually.) In several instances, I extracted the maps entirely from the book just so I wouldn't have to flip past them while searching for important information.
It's almost impossible to truly understand the layout of Rappan Athuk, but you can understand the logic and that's what matters.
In the book, there are levels, sections, and rooms.
Rappan Athuk's numbering scheme is structured around this taxonomy. For instance, 5-9 refers to level 5, room 9. And 10A-41 refers to level 10, section A, room 41.
Some editions (or all?) of Rappan Athuk feature chapters about a nearby town (Zelkor's Ferry) as well as the wilderness in the region. The wilderness map is big. There's a long road that stretches along the coast, some swamps, hills, and plains. There are 33 locations marked on the wilderness map.
It's a hex crawl. You can place your players at one end of the road, and then tell them to go for it. Find the dungeon you've been hearing so much about.
The thing is, it's not technically that hard to find Rappan Athuk. Many people in the region have heard about it, although few have approached it themselves. But there's a proper entrance, surrounded by a mausoleum, and it's visible from pretty far away and also people will tell you how to get there. It's not really a secret, and several people know a guy who knows a guy who knows a guy who found lots of treasure there. They won't go with you, but they'll tell you how to get there.
The "problem" with this is that there are so many other entrances to the dungeon that either your players are likely to stumble across one during the hex crawl, or else you might just arbitrarily want to start them at a side entrance.
My problem was that I kept forgetting which location on the map provided an entrance, because they are not well marked in the text. The text mentions each entrance, but it's embedded without keywords, boldface, or consistency within the description of some portion of the location. It's often easy to miss, and impossible to search for.
As a service to Dungeon Masters everywhere, here's an easy list with all entrances to Rappan Athuk from each location of the map. Not all maps, from what I understand, have all locations, and I'm not entirely sure whether the numbering is the same across releases, so I include the label as well:
Mark up your book with red ink, or highlight the level and section numbers, because they're easy to miss!
Image by Frog God Games.