If it isn't limestone or another cave-forming formation, you may have guessed it.
Underground bunker or bomb cellar.
Prohibition-era smuggler's hoard.
The famous "Mine-Shaft-Gap" from Dr. Strangeglove.
Please reassure me that it isn't giant, mutant Mole-Men!
Or some fracking side-effect.
Has the local community been pumping ALL of its ground water out of the ground, dropping the water table dramatically? I know some desert towns have done that. Build, build, build, pump, pump, pump themselves dry, then blow away. That might cause some subsidence, but again I would expect local newspapers to be blaming someone, if nothing else.
