 | Roman legend says that mortals could access the underworld at certain points on Earth. Located across the Mediterranean, these “gates of hell” were marked by stone passageways built over geologic features like hot springs or caves. In displays of supernatural power, ancient Roman priests would lead an animal through the passageways—an act that killed the creatures, but left the eunuchs unharmed. Researchers say they’ve discovered how these gates work. The Hierapolis gate is still deadly to this day. Locals report finding dead mice, cats, weasels and even foxes at the site. So how did the ancient priests survive? |  |
 | Hierapolis’ gate is positioned on a fault. Fissures emit a steady stream of volcanic carbon dioxide. Though the gas is harmless in limited quantities, clouds of CO2 can swiftly suffocate any living creatures that pass through. Researchers measured the CO2 concentration at various heights over time, and found that the concentrations of gas differ during day and night. With the sun overhead, the clouds of CO2 dissipate. But at night, the gas collects, forming a thick layer. |  |
 Trajan's Column in Rome |
The concentrations grow high enough overnight that they could kill a person within a minute, according to the study. Since the clouds of CO2 streaming up from the fissure are denser than air, it collects at ground level. This means that sacrificed bulls or rams, whose heads were too short to reach above the deadly layer of gas, would swiftly die. But the priests were likely tall enough to avoid death. |
