Located in the ancient city of Hierapolis (Turkey), it was once a site dedicated to the Roman god of death, Pluto. Ancient historian Strabo visited the place and said that “Any animal that passes inside meets instant death. I threw in sparrows and they immediately breathed their last and fell”. After discovery in 1965, Pluto’s Gate’s dangers were proven to be more than just a myth. Scientists measured the CO2 concentration and discovered that while during the day the sun dissipates the gas, at night, when the temperature drops and CO2 becomes heavier than air, it pools at the bottom forming a deadly “lake”. At dawn, the CO2 concentration 40 centimeters above the arena floor reaches 35%, which is enough to kill animals and even people within minutes. Luckily, the concentration drops significantly with height, so standing by the “gates” would only pose a threat to smaller animals.