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Implied Mega-dungeons of the Known Worldby Mike Harvey
Mystara has plenty of implied megadungeons.
Koriszegy Keep (aka the Haunted Keep) -- this is just crying out for megadungeon treatment. It's a legendary site filled with undead, surely ruled by a Nosferatu Lich. You could probably use Castle Zygag or Greyhawk Ruins for this, since it obviously has a ruined castle on top.
Fort Doom - there has to be a very large dungeon beneath this. The baron needs barracks to house his humanoid armies, and Bargle needs rooms for experiments. Not to mention the ancient Traladaran cult temple that exists even further down...
The Lost Dwarf City beneath Adrian Peak, in the mountains northwest of Wereskalot. The location is marked on ancient maps, but is not mentioned in the Gazetteers...
The Fortress of Azazabus beneath Mt. Dread. Use Undermountain for this? They are both located in mountains, both built by an evil wizard.
The endless catacombs of Barimoor.
Itheldown Island is begging for a sprawling (and high level) complex beneath it.
The Wurld Below (broken lands) and the Shadow Elf Caverns. Not properly a dungeon but definitely mega!
Lower Dengar. Just how deep do the dwarven mines go? We know that they extend through the dwarfgate mountains and eventually connect to the Shadow Elves' realm thousands of feet below the surface. That has got to be one of the largest megadungeons of them all.
I put an ancient Nithian megadungeon in the mountains of far western Ylaraum (Makistan) that connected with the shadow elf caverns.
Skull Mountain. Not sure if this qualifies as a megadungeon, but could drop this just about anywhere.
Stonehell. This feels like maybe a former Thyatian or Alphatian prison. Somewhere no longer under imperial control. Perhaps in the mountains of Ylaruam. Or maybe Darokinian, located in an obscure borderland region.