Pluto's Gate at Hierapolis

The ancient city of Hierapolis was best known for its Plutonium, or gate to the underworld, a cave from which issued a steaming spring and fumes that were said to be deadly to all who approached. There was a sort of enclosure in front of this cave, of which Strabo (64 B.C.- 24 B.C.) wrote,

This space is full of a vapor so misty and dense that one can scarcely see the ground. Any animal that passes inside meets instant death. I threw in sparrows and they immediately breathed their last and fell.

Archaeologists have been exploring the site, in modern Turkey, for a century, but they have only now located the Plutonium. They found it by following the steaming spring to its source. According to the discoverers, the cave is still fatal to birds that fly too close:

We could see the cave's lethal properties during the excavation. Several birds died as they tried to get close to the warm opening, instantly killed by the carbon dioxide fumes.

But in the interests of science I will note that those discoverers are Italian, so they may have gotten over-excited and exaggerated a little.

I find it interesting that in Roman times Hierapolis prospered as a health spa. The waters that were so deadly when they emerged from the mountain were, a few yards away, channeled into elaborate baths where wealthy visitors could take the waters. This seems in keeping with one of the principles of ancient medicine, that poisons and medicines were closely intertwined, so that what was a medicine at one dose or in one circumstance would be a poison at another. The deadlier the spring, the more medicinal its waters. A visit to the site thus combined three of the ancient world's favorite pastimes: health, bathing, and pilgrimage. The site lost its main attraction in the late fourth century, when Christians filled the Plutonium with stones, but the city endured into medieval times. In 1354, it was crushed by an earthquake. Since the spring that had been its center no longer mattered, the city was rebuilt on a level site a few miles away, become the modern Turkish town of Pamukkale.

The ancient springs are still a tourist attraction, but for geology buffs rather than hypochondriacs. The region is famous for the travertine formations, much like those at Yellowstone in the US.