The World's Largest Dungeon is a Dungeons & Dragons adventure set entirely in an enormous dungeon. It is over 800 pages long and was produced by Alderac Entertainment Group in 2004. It also includes 16 full-color poster maps, making its single tome one of the largest campaign settings in one product.
The players can't leave the dungeon the way they entered because the entrance uses a one way wall of force effect. This means that all supplies must be taken into the dungeon with them. The campaign is designed to take four to six characters from first level to 20th or above over the course of two real-time years and at least a year of game time. The dungeon contains examples of every type of monster in the System Reference Document  as well as a few new ones.
Several Blogs have been established to chronicle adventures in the dungeon, which Alderac collected on their website.
The World's Largest Dungeon was also parodied in Knights of the Dinner Table as the Biggest Damn Dungeon Ever which was a product by the fictional creators of Hackmaster, but unlike the World's Largest Dungeon, it was only an alphabetical collection of monsters.
In 2006, the Guinness Book of World Records listed World's Largest Dungeon as the most expensive roleplaying book to date. And while this has since been eclipsed by several books, it remains the largest role-playing book ever published (at 840 pages).