The Tsarichina Hole case in Bulgaria is usually discussed as a paranormal military story, but I think it may fit just as well into alternative history because of one overlooked possibility:
what if the military believed they were clearing an already existing underground structure, not simply digging a brand-new tunnel?
From 1990 to 1992, the Bulgarian military conducted a secret excavation near the village of Tsarichina, about 30 km from Sofia. The operation reportedly lasted more than two years, cost millions, involved restricted access, and ended with the entrance being sealed in concrete.
by No_Money_9404