Cave of Dogs

Illustration of the cave from a 1865 science book. A guide shows a suffocated dog to a man and woman tourist.
Lake Agnano and the Cave of Dogs depicted in an etching by the Sieur de Rogissart, 1706
The principle of the Cave of Dogs sketched by Alfred Swaine Taylor, 1832. The carbon dioxide (marked with a darker color) gathers in the lower part of the cave.

The Cave of Dogs (Italian: Grotta del Cane) is a cave near Naples, Italy. Volcanic gases seeping into the cave give the air inside a high concentration of carbon dioxide. Dogs held inside would faint; at one time this was a tourist attraction.