This cat's vision of the undeworld is pretty much perfect; I want to play in his game.
Reading his entries had me thinking about the vast subterranean vaults of pulp fantasy as the megadungeon, which led to me thinking of the underworld as a wilderness.
If you have a dungeon/underworld complex that spans the tens of thousands of feet, and you use the snails-pace oldschool D&D underground travel rates, you end up in a situation where it takes ~1 day of travel to cover ~1 mile.
Suddenly bringing food and water is even more important.
I love the idea of a megadungeon composed of a complex of miles-spanning caverns and tunnels, connecting various dungeon complexes.
Journey to the Center of the Earth; Descent into the Depth of the Earth; The Descent (novel); mega-spelunking. Caches. Base camps.