He covers many of the essentials — including The Temple of Elemental Evil, Undermountain, and Castle Greyhawk — and throws in a few clever suggestions I hadn’t thought of (such as the Death Star and Minecraft.) He here is on the modern classic Blackreach:
Blackreach is a signature location in Elder Scrolls: Skyrim. It’s semi-abandoned underground city filled with eerily beautiful glowing mushrooms, strange ruins, rare plants and other oddities. The first time you delve into Blackreach, you can’t help but be a bit awestruck. The silence down there is intense, creating a tension and wonder I’ve rarely experienced in video games. Blackreach itself is massive, but to get to it you actually have to work your way down through another dungeon, a Dwemer ruin. And Blackreach is actually connected to three of these dungeons, so there’s no doubt it’s worthy of the “mega” appellation.
As thorough as he is, there’s plenty left over for folks in the comments section to add, including the epic Rappan Athuk, Dragon Mountain, Judges Guild’s early classic Tegel Manor, Arduin Dungeon, Ultima Underworld, and many others.
I’m disappointed that so far no one has mentioned a few of my favorites, including Monte Cook’s massive (and now extremely hard to find) Ptolus, Gygax’s Castle Zagyg modules, AEG’s ridiculously oversized World’s Largest Dungeon, and Goodman Games’ massive Castle Whiterock. Ah well. Who would have thought the market would be crowded with megadungeons?
You can read Ed’s complete io9 article here.