There are numerous caves named dog cave or dog's cave because of a story or legend with a dog and the cave. Sometimes there were dead dogs found inside, sometimes a dog got lost and was saved, sometimes a dog saved someone lost inside the cave. That is not what we are talking about.
|Image: soap bubbles swim on the carbon dioxide.|
A dog cave is a cave where a person entering the cave does not sense any difference, while his accompanying dog collapses. The dog dies in a few minutes, if it is not brought back, out of the cave. If brought back in time, the dog soon recovers and seems unharmed.
This effect was noted many years ago, and the most prominent spot is also the oldest known such cave: Grotta del Cane in Italy, near Napoli. It was described by Pliny the Elder in Roman times, Johann Wolfgang von Goethe and Andre Dumas. While this disturbing effect was a source of fear in old ages, it became a tourist attraction during the 19th century, when people from northern Europe started modern tourism by travelleing to Italy. Today the dog caves are almost forgotten.
Now for the geologic background. Actually a dog cave is not a cave type, in most cases it is not even a cave. Typically it is more or less artificial, a quarry or cellar. It areas with volcanism, the magma underground produces various gases, mainly carbon dioxide (CO2) and steam. The carbon dioxide often gets dissolved by ground water, and when the water reaches the surface it is called mineral water with this nice bubbling. If the carbon dioxide reaches the surface as a gas, it forms sources of carbon dioxide gas, which mostly go unnoticed, except if they bubble up from a lake floor or inside a confined space. In a confined space, there is no wind, so the carbon dioxide, which is heavier than normal air, forms a sort of gas lake at the floor of the cave.
And that is the reason for the effekt with the dog: there is almost no oxygene at the floor, so the dog suffocates. But as carbon dioxide is not poisonous, the animal recovers as soon as it gets enough oxygene. People do not mention the gas, as they are too high. They would also get unconscious if they would lie down.
Carbon dioxide is invisible, but it is possible to visualize the surface of the carbon dioxide lake with soap bubbles. The bubbles contain normal air, which is lighter than pure carbon dioxide, so the bubbles swim on the lake.
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