Interesting Factoid: Hierapolis means "sacred city"...or "a holy city." It's mentioned in the Bible by the Apostle Paul:

Colossians 4:13
For I bear him record, that he hath a great zeal for you, and them that are in Laodicea, and them in Hierapolis.

Even though it was thought to have an entrance into the underworld in the Greco-Roman period...it was a thriving Roman city that became an important Christian center.

Shrouded in misty poisonous vapors, Pluto’s Gate, or the Plutonium, was a cave entrance sacred to Pluto, the Roman god of the underworld. According to the first-century geographer Strabo, the site was home to rituals in which any animals entering the enclosure “meet with sudden death.” Hierapolis archaeologist Francesco D’Andria reconstructed the route of the area’s thermal spring to discover Pluto’s Gate, which was destroyed by Christians in the sixth century. The Plutonium’s infamous mystique is not just the stuff of legend; during the excavation, several birds were killed by carbon dioxide emissions as they approached the Plutonium cave’s entrance.
Source

@sweetsssj Please stay away from those vapors!!!