When wealthy travelers embarked on the Grand Tour of Europe in the 17th and 18th centuries, one popular stop near Naples was the Cave of Dogs. Here, local guides would happily demonstrate the cave’s accumulation of carbon dioxide by suffocating dogs in the fumes, before hopefully reviving them in the waters of the nearby lake.

The Cave of Dogs (Grotta del Cane) is a small cave near Pozzuoli, Naples. Inside is a fumarole that spews out carbon dioxide, which, being heavier than air, accumulates in the lower, deeper parts of the cave. Back in its heyday, visitors would typically visit this strange cave as part of a tour of the volcanic Phlegraean Fields.

Local guides, more concerned with healthy tips than healthy animals, would demonstrate the cave’s unusual toxicity by forcing dogs to enter the low-lying carbon dioxide until they suffocated and passed out. To revive them, the guides would throw the unconscious canines into the cold waters of the nearby Lake Agnano.

The earliest reference to the Cave of Dogs is found in Pliny the Elder’s Naturalis Historia, a monumental text dating back to 77 AD. But the cave’s popularity surged far later with the rise of the Grand Tour, during which time notable luminaries such as Goethe, Alexandre Dumas, and Mark Twain entered the toxic cavern. These humans, being taller than dogs, could enter the cave unaffected, while the unfortunate dogs would soon suffocate in the ground-hugging gases.

Mark Twain, incidentally, was particularly keen to see the cave, and strangely eager to kill a dog. In his travelogue The Innocents Abroad, he wrote: “I longed to see this grotto. I resolved to take a dog and hold him myself; suffocate him a little, and time him; suffocate him some more and then finish him. We reached the grotto at about three in the afternoon, and proceeded at once to make the experiments. But now, an important difficulty presented itself. We had no dog.”

The first person to do a thorough scientific study of the cave was Athanasius Kircher, a German Jesuit scholar and polymath. Kircher visited the cave in 1638 and made a series of experiments and observations. After witnessing a dog forced into the cave, he wrote:

“A dog was introduced to the cave. When its head was above the cited level [noted in a diagram he made of the cave], nothing happened. But when its head was lowered, the dog tried desperately to get away. When tied to a club to keep it close to the bottom, it seemingly died. When plunged into the lake, it showed a few signs of life and when pulled to the shore it rose, looked around and ran away to avoid a repeat performance.”

Such grim spectacles thankfully came to an end in the 1870s, when the lake became polluted and was subsequently drained. Tourism to the cave dropped off almost completely and it was closed to visitors. The local dog population, meanwhile, breathed a collective sigh of relief.

The Cave of Dogs fell into relative obscurity until fairly recently, when local volunteer groups decided to clear up the area and improve access to the cave and other local attractions. They also reestablished guided tours to the cave, but presumably with no dogs allowed.