What dungeon, you ask? Dragon’s Delve — a massive 27-level mega-dungeon more than half-a-million words long! It’s an adventure with 661 encounters, none of which are empty rooms or featureless corridors, arranged over nearly 30 maps! A whole campaign worth of material, capable of taking characters from 1st to 20th level, that begins with a bell on a string set by goblins and ends with a battle against a great wyrm red dragon demigod where the fate of a god hangs in the balance!
And while Dragon’s Delve may not be the biggest dungeon in the world, it has the distinction of having been conceived by Monte Cook (Ptolus, 3E Dungeon Master Guide) as the initial offering for Dungeonaday.com — a subscription website that created a classic “dungeon delve” style dungeon by releasing a single hyperlinked, cross-referenced encounter every weekday (each of which now has years of forum posts giving ideas on how to tweak, improve, or survive its challenges).