Hierapolis, Laodicea, and Colossae are biblical cities spoken of by the apostle Paul. 

Hierapolis (modern Pamukkale) did not have an apostle's letter written to it, but in the second century, it became an important city. The apostle Philip was martyred there, and a disciple of the apostle John, Papias, lived there. 

One of the apostle Paul's letters was addressed to the church at Colosse due to the influence of some incipient gnostic beliefs, and the man Philemon appears to have been a member of that church. The apostle John addressed one of his letters in the Revelation to the church at Laodicea, speaking of their lukewarm condition.

Hierapolis (Pamukkale) and Caesarea Philippi (Panias, Banias) in Israel both have what was considered an entrance to Hades, though the entrance in Turkey boasted the fumes of Hades. Hierapolis was where the apostle (not evangelist) Philip was martyred and where Papias was bishop in the second century. The poison emitting from the site in Hierapolis would kill someone going into it and the priests would demonstrate this by putting in animal sacrifices, yet they did enter very briefly, and held their breath, I would presume. Colosse (it is finally being excavated) is near Laodicea and Hierapolis, though the latter is on the other side of Lycus Valley and Laodicea on the same side and much nearer to Colosse. The back area of Colosse has cold running waters in the streams, which also overflow on the ground near the city of Colosse. The rusted pipes of Laodicea demonstrate the hot pipes of the city and illustrate the statement that John uses, that Laodicea has neither the refreshing cold waters of Colosse nor the warm healing waters of the hot springs at Hierapolis.