See this web site: http://www.termestufedinerone.it/en
These wonderful, historic baths are near Pozzuoli, about 2 hours drive south from Roma. The baths are for the local people and as a visitor you can only gain access by staying at a local hotel as you become “resident” for a day and then you can go, paying about 30 Euro per person.
Most visitors would NOT choose to go as from the outside it looks average and the staff do not generally speak English. But once you get in and everything is explained to you, you will not want to leave. This area is built on a series of “faults” and hot water, indeed boiling water which comes out of these faults can reach 70 degrees. The Terme di Nerone are right next to the sea so the natural spring water is also mixed with sea water. The natural sauna in the rock, inside the terme, is 54 degrees at the entrance and then seems to get even hotter as the heat reflects off the stones if you dare to go further inside. The big outside swimming pool is at 35 degrees (25m x 10m) and the smaller indoor pool is at around 40 degrees.
The place is amazing, one seems to lose the concept of time inside and there is also a snack bar serving excellent and very inexpensive food, with seating both indoor and out.
It is not too easy to find nor to get in, so Services can help you, note that you require about 3 hours to really appreciate the experience. NOT to be missed.