Grotta del Cane

Grotta del Cane ('Grotto of the Dog'), a small cave near Naples, in the vicinity of Lake Agnano and of Puzzuoli, contains carbonic acid gas with 77 per cent. of carbonic acid. This cave was known to the ancients, and is described by Pliny. It derives its name from the practice of introducing into it small dogs, which are soon almost deprived of life by the gas that owing to its density clings to the floor of the cave; but they soon recover upon being restored to the open air.