The discovery was made at a site in Turkey, known as Pluto’s Gate – or Plutonium in Latin – and was fabled as the doors to the underworld in both Greek and Roman traditions. Just as described in Greek mythology, the cave emitted poisonous gasses, and was described as “lethal” by the researchers who discovered it. Francesco D’Andria, professor of classic archaeology at the University of Salento, told Discovery News in 2013 that the cave killed birds trying to enter. He said: “We could see the cave’s lethal properties during the excavation. Several birds died as they tried to get close to the warm opening, instantly killed by the carbon dioxide fumes.”

The gates were part of a much larger dig site around the ancient city of Hierapolis.

Austin Considine at Vice, explained that “the Ploutonion is really a natural phenomenon, an opening in the earth’s crust, like a cave, from which foul and poisonous gasses escaped – also known as ‘mephitic’ gasses.”

Mr D’Andria added: “We found the Plutonium by reconstructing the route of a thermal spring. Indeed, Pamukkale’ springs, which produce the famous white travertine terraces originate from this cave.”

He added that even in ancient times the gate attracted a lot of interest, and was even a tourist attraction.

Those who travelled to the gate were given birds to test the gasses as priests sacrificed animals to gods of the dead.

D’Andria continued: “People could watch the sacred rites from these steps, but they could not get to the area near the opening. Only the priests could stand in front of the portal.”

The site in Turkey was destroyed by Christians and later by earthquakes all the way back in the sixth century.

There are other such “gates” across the world, although none have the same specific cultural significance as the one near Hierapolis.

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Once at the bottom, he collected soil samples as he aimed to learn more about the tough conditions at the bottom of the crater to see whether life could survive there.

After the trip, Korounis told National Geographic: “‘Surreal’ isn’t a strong enough word. When you go out over, looking straight down, it’s literally like another planet almost.

“I mean it was scary stepping over the edge, but when you’re at the bottom, it’s just so beautiful.

“I described it as a coliseum of fire – just everywhere you look it’s thousands of these small fires. The sound was like that of a jet engine, this roaring, high-pressure, gas-burning sound.

“You feel very, very small and very vulnerable in a place like that.”