The Cave of Dogs 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 CO2 gas, being denser than air, tends to accumulate in the deeper parts of the cave. Local guides, for a fee, would suspend small animals inside it—usually dogs—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. Famous tourists who came to see this attraction included Goethe, Alexandre Dumas père, and Mark Twain. The lake became polluted and it was drained in 1870; the spectacle fell into disuse and the cave was closed. However the area is now being restored by volunteers.
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. A video of this has been made.
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