The wordy stuff...
The Solfatara site is privately run and entry costs €7 for adults, with a reduction of 20% for holders of the Campania Artecard:
Allow perhaps an hour for your visit to the crater - longer if stopping at their pleasant bar/cafe, and having a look in the small museum of scientific instruments and early seismological monitoring equipment etc.
At the port in Pozzuoli, catch the CTP line's hourly P9 bus - from its stop, on the far side of the vehicle lanes that lead onto the ferries....
If coming from Naples, note that the 152 bus mentioned on their "How to reach us" page was dropped in May 2011... whilst the services previously operated by Sepsa now come under EAVbus.
For their timetable, look for the "Napoli - Monte di Procida" link, here:
Otherwise, the entrance to the solfatara site is an easy walk of about half a mile / 800 metres up from the "Pozzuoli Solfatara" station, which is on Metro line 2.
Sulphur and antimony deposits, Pozzuoli solfatara
The volcano known locally as the solfatara (literally 'land of sulphur') was formed some
4000 years ago, and last erupted in 1198.
On humid days like this one in June, the clouds of steam issuing from the fumaroles are
particularly impressive - as are the bright orange deposits of sulphur and antimony that
crystalise out from the vapours.
For opening times etc, see... http://www.solfatara.it/vulcano/en/index.php
Boiling mud in the Solfatara volcano
Hell and Purgatory
Two ancient steam chambers - stufe - for patients wishing to inhale the sulphurous fumes, once thought to be healthgiving!
The Bocca Grande (great mouth)
The largest of the fumarole, vents from which superheated steam breaks free,
carrying sulphurous fumes into the air which deposit elemental sulphur and
The solfatara volcano - clouds of superheated steam issuing from one of the vents
Deposited red sulphur crytals