Two millennia ago, a small Greco-Roman temple in present-day Turkey awed and enthralled its residents. Just beyond its stone gate, in a grotto shrouded in a heavy mist, a strange force worked dark deeds: Bulls ushered inside would lie down and perish; the castrated priests in charge would emerge unscathed. 
 
 Was it the bloodthirsty will of Pluto, the god of the underworld? The supernatural power of the priests? New research published on Feb. 12 in the journal Archaeological and Anthropological Sciences suggests a far earthlier explanation to the cave’s mystery: noxious carbon dioxide. 
 
 Using a portable gas analyzer, volcano biologist Hardy Pfanz led a team of scientists to discover that vapors emitted from the mouth of the cave—belched from a fissure running deep beneath the area—reached levels of 4 percent to 53 percent volcanic carbon dioxide, depending on the distance from the ground. The lower to the cave floor, the higher the amount of suffocating gas, which formed a lethal lake of carbon dioxide. (The noxious gas is heavier than oxygen, so it settles lower, which is one reason CO₂ leaks in your house make basements deadly.) Animals with noses to the ground likely breathed in far more gas than the humans walking upright beside them, which could explain the priests’ miraculous imperviousness. 
 
 View of the ruins of the city of Hierapolis, near modern-day Pamukkale, Turkey. Photographer: alla_iatsun/iStockphoto 
 
 Although rediscovered only in 2013 near the town of Pamukkale—famous for its surreal, Unesco-designate travertine hot spring terraces—the cave’s existence has been known since antiquity as part of what was then Hierapolis. Known as “Plutonium” after Pluto, it was thought to be a gate to the underworld and a way to convene with the god by offering animal sacrifices. Spectators would watch in disbelief from a nearby arena. A description written by the Greek geographer Strabo, who lived from 63 B.C. to 24 A.D., makes a great deal more sense given what we know today: “This space is full of a vapor so misty and dense that one can scarcely see the ground … bulls that are led into it fall and are dragged out dead,” he wrote. But although priests entered and left the cave unharmed, Strabo noted that they would “hold their breath as much as they [could]” and displayed “an indication of a kind of suffocating attack.” 
 
 The terraces of Pamukkale. Photographer: gloriasalgado/Room RF 
 
 Two thousand years later, visitors should still be wary of the gate; during the 2013 excavation, archeologists witnessed several birds drop dead after flying too close. 
 
 Granted, merely getting here might prove a challenge. Beginning in 2015, Turkey’s rough relationship with Russia has tanked its tourism industry, which despite a brief resurgence last year continues to suffer following tensions with the U.S. government. In December, the U.S. and Turkey mutually suspended visa services amid a feud over the arrest of a local employee of the U.S. Consulate in Istanbul. Services later resumed, but the U.S. government urged its citizens in January to hold off on travel plans to the region, citing terrorism concerns. 
 
 The silver lining? Fear generally keeps tourist crowds at bay. Should you visit, you might have the plutonium all to yourself. ||||| Abandon hope, all ye who enter here. Hieropolis, in what is now Turkey, features an entrance to 'hell' that was once the site of ritualistic, mystical sacrifices. Flickr;F. Tronchin /CC BY-NC-ND 2.0 
 
 Around the second century BCE, the kings of Pergamon built a thermal space at Hierapolis, in what is now modern-day Turkey. Now, it lies in ruins, but something within still stirs – something far more ancient than those that once lived atop it. 
 
 Seven years ago, a “gate to the underworld” – also known as Plutonium, named after the Roman god of the underworld – was discovered there, emitting a curious haze that proved to be lethal to anything that stood inside the cave for too long. It’s known that humans that entered the cave fared better than the bulls, rams, and songbirds they often sacrificed, and a new paper has finally revealed why. 
 
 Initially spotted by ScienceMag, the paper – in the journal Archaeological and Anthropological Sciences – explains that the cave sits over a fissure that continuously releases pent-up geologic carbon dioxide in considerable quantities, even today. The concentration of carbon dioxide drops off with distance from the ground, though, which is why animals in the cave died but the taller priests who brought them into it often survived. 
 
 The team, led by the University of Duisburg-Essen (UDE), explain that stories of ritualistic sacrifices in the cave appear in the writings of several long-gone scribes. Strabo, a Greek geographer, once recalled: “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.” 
 
 Rather nonchalantly, he added: “I threw in sparrows and they immediately breathed their last and fell.” 
 
 Although the emergence of volcanic gases was known by this point, the team wanted to better understand the fine details of this sinister portal. Taking careful measurements over time, they also found out that the sunlight and wind during the day helped disperse the deadly mist. 
 
 You can visit Hieropolis today. Michael Gaylard/Flickr; CC BY 2.0 
 
 This makes the hours before dawn the riskiest time to enter the cave. At night, concentrations were so high that they “would easily kill even a human being within a minute.” 
 
 “These emissions are thought to reflect the Hadean breath and/or the breath of the hellhound Kerberos guarding the entrance to hell,” they add. 
 
 The Eunuch priests were savvy enough to know how the concentrations changed throughout the day, which is why the team surmises that sacrifices took place when they were highest. Emerging from the cave seemingly unharmed, their “godlike” powers became the stuff of legends. 
 
 “The Galli [Eunuch priests] stood on stones around the poor bull or goat and demonstrated their supernatural powers,” lead author Prof. Hardy Pfanz, a researcher in volcanism and biology at UDE, told IFLScience. “At this height they could stand for 20-40 minutes without being endangered.” 
 
 Independent of these stones, “nobody could enter the gate to hell without getting asphyxiated,” but “if the Galli kept their breath for a while they could crawl into the gate up to their waist.”
– Centuries ago, bulls, rams, and other animals led into an ancient cave for religious ceremonies died of seemingly mystical causes, while the priests accompanying them suffered no such fate. Scientists say they've now figured out the secret behind this "Gate to Hell" in the ancient city of Hierapolis, per Bloomberg, and it has nothing to do with gods of the underworld. Instead: carbon dioxide. A study in Archaeological and Anthropological Sciences reveals that the cave located in modern Turkey, which was rediscovered in 2013, sits over a fissure that releases the gas. Volcanologist Hardy Pfanz and his team used a portable gas analyzer to figure out the phenomenon. So how did the priests survive? Because carbon dioxide is heavier than oxygen, it sinks—meaning animals closer to the ground succumbed, while the humans were OK. Or mostly OK: Ancient accounts depict the priests sometimes emerging and struggling for breath as spectators watched from a nearby site. IFLScience explains that the old-time priests likely understood that concentrations were diluted during the daytime by sunlight and wind—and so they held sacrifices before dawn for a quicker kill—and were also smart enough to climb up on stones around the doomed animals to show off their supposed powers. "At this height they could stand for 20 [to] 40 minutes without being endangered," Pfanz says. Without the stones, he adds, "nobody could enter the Gate to Hell without getting asphyxiated." (An incredibly old human fossil was recently found in a cave in Israel.)