A cave ancient Romans believed to be a gate to the underworld was so deadly that it killed all animals who entered its proximity, while not harming the human priests who led them. Millennia later, scientists believe they have figured out why - a concentrated cloud of carbon dioxide that suffocated those who breathed it. Dating back 2,200 years, the cave was rediscovered by archaeologists from the University of Salento back in 2011. It was located in a city called Hierapolis in ancient Phrygia, now Turkey, and it was used for animal sacrifices of bulls led through the Plutonium -...