Field of Science

The Dog´s Cave, Animal Cruelty and an unseen Volcanic Killer

The Grotta del Cane (the Dog´s Cave) is a short, 9m deep, artificial gallery located on the rim or the basin of Agnano near Naples. The basin is in reality the remains of an 8.000-4.000 year old collapsed volcano, just one smaller caldera of the Phlegraean Fields.

Fig.1. The Phlegraean Fields with the caldera of Agnano located almost at the center, from SUESS, E. "Das Antlitz der Erde" (1892).

Since antiquity the area is known for it´s volcanic activity, like hot springs and fumaroles, called mofettes.
It´s also famous because inside the dog´s cave small animals like dogs and bird will slowly suffocate. Already the Roman poet Vergil (70-19 BC) writes of the nearby lake of Averno:

"There was a deep stony cave, huge and gaping wide,
sheltered by a dark lake and shadowy woods,
over which nothing could extend its wings in safe flight,
since such a breath flowed from those black jaws,
and was carried to the over-arching sky, that the Greeks
called it by the name Aornos, that is Avernus, or the Bird-less

Naturalists from Antiquity until the Renaissance know of the strange phenomenon and speculated about toxic vapors coming from the underground, killing smaller animals. German polymath Athanasius Kircher (1602-1680) suggested that “Ochre, Sandrak*, Orpiment and Arsenic” compounds, known at the time to be toxic (and very popular as pigments to paint), were to blame.

In the early 19th century naturalists in self-experiments determined that below a certain level in the cave they experienced a strange dizziness and started to lose feelings in their legs. Those were recognized as symptoms of a carbon-dioxide intoxication. The invisible and odorless carbon-dioxide is heavier than normal air and tends to flow to the bottom of wells and caves. 
The gas emanates from the volcanic underground and accumulates in the dog´s cave. At the entrance the layer of carbon-dioxide is just sufficiently thick to suffocate smaller animals, deeper in the cave it can become dangerous also for humans. 

The dog´s cave became a popular and somehow macabre tourist attraction in the 18th-19th century. Visitors could buy a dog from a nearby shelter and observe how it would become unconsciousness and suffocate inside the cave – when not saved in the last moment by throwing it in the nearby lake of Agnano (today dried up). The cold water would revive the poor dog in most of the cases...

Fig.2. Demonstrating the deadly effects of the dog´s cave, from “The book of curiosities: containing ten thousand wonders and curiosities of nature and art” (1822). 

*Sandrak is actually a resin used to fix pigments


KROONENBERG, S. (2013): Why Hell Stinks of Sulfur: Mythology and Geology of the Underworld. University of Chicago Press: 352

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