Kingdom of the Wheel from my older posts -- the idea of well-known megadungeons, with regular parties of adventurers mapping segments of it, sharing it with several rival mapping guilds, contributing to a dungeoneering town or city that has sprung up around the economy of adventurers and their quirks.
To that end, I encountered a new resource -- www.megadungeons.com -- which only started up this year. I also rediscovered a post from Blood of Prokopius about a solo game called How To Host A Dungeon which, sadly, seems to only be available in PDF form to those of us living outside of the Americas.
I've also been eyeing Doomed Slayers by Jurgen Hubert for a neat take on the culture of adventurers -- one that may not necessarily be adopted country-wide, but certainly take effect in the areas near these megadungeons.
I've also been looking at my old purchased megadungeon material... not sure where I'm going with this yet, but there's interest, and I'm hoping that going through this stuff will trigger something.
There are certainly products that have come out in the past about cities with megadungeons beneath them -- Undermountain is one. In fact, in the heyday of the D20 craze, there was a 'dungeonworld' type supplement. In the OSR realm, there was even Zak S.'s gigacrawl megadungeon effort, and the completed 'Greyhawk' levels, and various experiments in megadungeon design and construction. And let's not forget those fabulous web-based geomorphs online.
So what is it? What are you trying to tell me, brain?