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Why Dogs Can’t Stand ‘The Cave of Dogs’

In the west of Naples, Italy, is a large volcanic area called Phlegraean Fields filled with craters of old, extinguished volcanoes. Lying mostly underwater, the area is still volcanically active as evidenced by the numerous boiling pools of mud and fumaroles from which copious amount of steam can be seen rising at any time of day or night.

Centuries ago, travellers who could afford to travel came to Naples to see the famous volcano that buried the Roman towns of Pompeii and Herculaneum under magma and ash. They also made a visit to Phlegraean Fields, where tour guides took them to a small cave called “Cave of Dogs”, or Grotta del Cane, for a gruesome little experiment.

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A guide shows a suffocated dog to two tourists at the Cave of Dogs near Naples.

The cave’s entrance is a narrow opening on the side of a hill, that leads to a short corridor—about ten meters long—that slopes downward ending in a cavity. Inside this cavity is a fumarole that releases carbon dioxide, which being heavier than air settles at the bottom of the cavity forming a shallow lake about 30 centimeters deep. The carbon dioxide lake doesn’t affect people because their heads are well above its surface, but for something much shorter, such as a dog, the condition inside the cave can be fatal.

Carbon dioxide is a normal component of air, and the low concentration at which it exists is harmless to humans and other living beings that breathe oxygen for survival. But as the concentration of carbon dioxide in air increases, it leads to serious physiological harm—a condition known as hypercapnia. Initially, carbon dioxide poisoning causes flushed skin, muscle twitches, elevated blood pressure, and reduced neural activity. Severe hypercapnia causes headache, lethargy, increased cardiac output, irregular heart beat, panic, disorientation, convulsions, loss of consciousness and finally death.

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In this 17th century etching of Lake Agnano and the Grotta del Cane, three men are seen taking a dog into the grotto to demonstrate the experiment in which noxious gases emitted from it cause asphyxiation of the dog. On the left, two local men suspend an asphyxiated dog in the lake in order to revive it.

To demonstrate the phenomenon to tourists, in earlier times, local guides would take a dog into the cave. The dog would be led into the pool of carbon dioxide, at which the poor animal would start to suffocate from lack of oxygen and soon lose consciousness. The collapsed dog would then be taken out of the cave and resuscitated by submerging it in the cold waters of the nearby Lake Agnano.

The cave was mentioned as early as Roman times, described by Pliny the Elder—who died in 79 AD during the eruption of Mount Vesuvius—but it didn’t became famous until about 400 years ago. This was when the dog experiments started taking place. Some of the famous tourists who saw this experiment included Goethe, Alexandre Dumas and Mark Twain.

In his travelogue, The Innocents Abroad, Mark Twain describes his visit to the Grotta del Cane:

Nero's Baths, the ruins of Baiae, the Temple of Serapis; Cumae, where the Cumaen Sybil interpreted the oracles, the Lake Agnano, with its ancient submerged city still visible far down in its depths-these and a hundred other points of interest we examined with critical imbecility, but the Grotto of the Dog claimed our chief attention, because we had heard and read so much about it. Every body has written about the Grotto del Cane and its poisonous vapors, from Pliny down to Smith, and every tourist has held a dog over its floor by the legs to test the capabilities of the place. The dog dies in a minute and a half—a chicken instantly. As a general thing, strangers who crawl in there to sleep do not get up until they are called. And then they don't either. The stranger that ventures to sleep there takes a permanent contract. 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 spectacle of the cave fell into disuse once Lake Agnano was drained in 1870. The cave no longer attracts tourist as it did in earlier times.

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A paintings by Camillo DeVito (1794-1845) shows a man dragging an unconscious dog away from Grotta del Cane. grotta-del-cane-2

The principle of the Cave of Dogs sketched by Alfred Swaine Taylor, an English toxicologist and medical writer, in 1832

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Lake Agnano and Grotta del Cane by Sieur de Rogissart, dated 1706.

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The cave as it appears today. Photo credit: Fiore Silvestro Barbato/Flickr

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