From Pluto's Gate Discovered: The Plutonium 'Gate To Hell' Found In Ancient City Of Hierapolis:
According to the Italian news agency ANSA, a team of archeologists working in the ancient Phrygian city of Hierapolis in southwestern Turkey claims to have located the Plutonium, or Pluto's Gate -- an ancient pilgrim site considered the entryway to the underworld. A small cave near the temple of Apollo, the Plutonium grew in association with death from deadly gases it emitted.
Francesco D'Andria of the University of Salento announced the discovery during a press conference in Turkey in mid-March, according to La Gazzetta Del Mezzogiorno.
D'Andria told Discovery News he also found remains of the temple, a pool used by pilgrims and a series of steps.
“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,” D'Andria added, according to Discovery News.
I love this stuff. Reading the article evoked many thoughts of Persephone and the rest. How terrifying the place must be when you see living creatures dying instantly by approaching it.