Keith Davies has had a series going for the past couple of days where he walks through creating a megadungeon. Rather than using a map, and filling in the spaces, he uses a node-based model. What that means is he is not as concerned with the exact map that connects the various locales, but rather, how are the different areas in the dungeon related. As my friends all like using minis, I find myself using this model as well, and in fact I often never have a complete map created. I want the dungeon they are exploring to feel massive, with the possibility of getting lost, and if they are mapping every 5′ square, it gets old quickly, and begins to feel very small.
I don’t think I’ll ever run a true mega-dungeon, but they are nice to have to have in a campaign world when you don’t have anything else planned. “Hey guys, wanna see what’s down that pit the last time you explored?”