Klaus WalterThe Plutonium of Hierapolis sits over fissures, from where such gases, especially carbon dioxide, enter the chamber. As carbon dioxide has a higher density than air, it will accumulate on the bottom of the cavity. In the early 19th century naturalists determined in self-experiments that below a certain level in the Dog's Cave they experienced a strange dizziness. The Dog's Cave became a popular and macabre tourist attraction during the 18th-19th century. Carbon dioxide flowing from the volcanic underground fills the pits, suffocating animals and humans foolish enough to enter such natural traps.

Source: Forbes February 25, 2018 18:27 UTC