When I first entered the hobby, I was greatly taken by the cross-section illustration of "Stone Mountain" (aka Skull Mountain) presented in the D&D Basic Set rulebook edited by Dr J. Eric Holmes. Indeed, it's not unreasonable to say that, for over three decades, Stone Mountain has been my mind's eye vision of what a "megadungeon" looks like. For those of you somehow unfamiliar with this illustration, here it is:
tent pole dungeon for long-term campaign use.
Much as like these cross-sections, they do have one problem: they're very stylized. That is, they're more art than map. That's fine for helping one to visualize the dungeon as a whole but it doesn't provide the nitty gritty details that are vital to ensuring that all the dungeon's pieces fit together properly. That's why I had Tim Hartin put together several images where all the levels of Dwimmermount were piled on top of each other to show how they connected. Here's one that shows seven different levels (1, 2A, 2B, 3A, 3B, 4, and 5):