D&D 5E seeking the Logical and Scientific for help with dungeon creation

[mflayermonk]

Dragon was caught in a tar pit, then the tar pit was covered in a landslide.

 

[Shadowdweller00]

I'd tend to go with imprisoned with powerful ritualistic magic by a cabal that didn't want to directly fight the dragon, or couldn't overpower it in straight combat. Possibly in a location that has since become home to a dragon-worshipping cult that is either siphoning off magical energy from the dormant dragon or wants to free the creature from its magical imprisonment. Or both.

 

[The Human Target]

Magic.