Not a natural cave but the flooded workings of one of the great old Lead Belt mines, and the largest underground lake in the world. You tour a mile of the abandoned mine by boat, past ore carts and tools left where the miners dropped them in 1962, and it is one of the top freshwater scuba destinations on Earth, diving in the clear, still, blue mine water. PUBLIC tours (walking and boat; scuba by booking), Bonne Terre, Missouri.
Missouri
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