The baths’ hottest room, where the temperature could rise above 40 or 50 ºC, and the air was full of steam. It would be close to what we now know as a “sauna”. Water was heated in a furnace housed in an adjoining room and was taken to the caldarium through a hypocaust, that is, an underground chamber connected to the furnace. The caldarium had a small, shallow pool, situated in the southeast corner of the chamber. As in Muslim baths, there were no steps for entering the pool.
A glazed ceramic duct embedded in one of the walls was used to circulate hot air from the hypocaust.
A number of archaeological studies performed in the late 20th century have revealed the complex construction of the caldarium, furnace and hypocaust. However, even today, we still do not know all the details about their operation.