Most fantasy worlds are post-apocalyptic, at least distantly so. How else would there be lost civilizations and powerful, world shaking artifacts? The D&D Known World/Mystara is the product of the destruction of Blackmoor, and suffers a second apocalyptic event if you play the world canon and subject it to the Wrath of the Immortals (I wouldn't - I have all these lovely Gazetteers to explore). The Warhammer World is also the product of a world changing accident stemming from technological hubris. In that case the Old Ones and the Slaan managed to tear a hole in fabric of reality and spill Chaos into the world.
But have a look at this - a warning to be left at sites of nuclear waste storage - and tell me this isn't the entrance to a super-dungeon:
This place is not a place of honor…no highly esteemed deed is commemorated here… nothing valued is here.
The danger is in a particular location… it increases toward a center… the center of danger is here… of a particular size and shape, and below us.
p.s. I probably owe a hat tip to someone for pointing me to this ages ago.