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, 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.