Cave of Dogs

The Cave of Dogs (in Italian "Grotta del Cane", literally "Cave of the Dog") is a small cave on the eastern side of the Phlegraean Fields near Pozzuoli, Naples. Inside the cave is a fumarole that releases carbon dioxide of volcanic origin. It was a famous, if gruesome, tourist attraction for travellers on the Grand Tour. The carbon dioxide gas, being denser than air, tends to accumulate in the deeper parts of the cave. Local guides, for a fee, would suspend small animals (usually dogs) inside it until they became unconscious. Because humans inhaled air from a higher level they were not affected. The dogs might be revived by submerging them in the cold waters of the nearby Lake Agnano. Tourists who came to see this attraction included Goethe, John Evelyn, Montesquieu, Alexandre Dumas père, and Mark Twain. The lake became polluted and it was drained in 1870; the spectacle fell into disuse (according to one source, it was banned before World War II for cruelty to animals). The cave entrance was blocked to prevent access by children.
The cave was often described in nineteenth century science textbooks to illustrate the density and toxicity of carbon dioxide, and its reputation has given rise to a popular scientific demonstration of the same name. Stepped candles are successively extinguished by tipping carbon dioxide into a transparent container.
In 2001 the cave was investigated by Italian speleologists including Rosario Varriale. Nine metres from its entrance the temperature was and the CO concentration was 80%, with negligible oxygen. It appeared to have been constructed in classical antiquity, possibly as a sudatorium; if so, the atmospheric conditions must have been markedly different.