Caliginous

Hierapolis is ſeated over-againſt Laodicea, where are to be ſeen baths of hot waters, and the Plutonium. [...] The Plutonium is under the brow of the hill, the entrance into which is no wider than that a man can thruſt himſelf through; yet it is very deep within, of a quadrangular form, containing about the compaſs of half an acre, and is filled with ſuch a thick and caliginous air, that the ground cannot be ſeen.