The Solfatara is a shallow volcanic crater about 2 kilometres from central Pozzuoli, part of the Campi Flegrei volcanic system. It has been continuously active for at least 2,000 years: sulphurous gases vent from dozens of fumaroles in the crater floor, the ground is unnervingly warm underfoot, and the air smells of rotten eggs. The Romans knew it as the entrance to Vulcan's forge and built temples nearby; the name "Solfatara" comes from the Latin for sulphur land.
The crater is not dormant. Carbon dioxide vents from the ground in concentrations high enough to asphyxiate animals, which led to the phenomenon the Romans described as the gates of the underworld — small animals that strayed close to the vents simply died without apparent cause. The science is straightforward, but walking across the warm, hissing ground with steam rising around you is still a genuinely unsettling experience. The Solfatara sits within a wider area of volcanic activity; the ground level of Pozzuoli has risen nearly a metre since the 1980s as the magma chamber below pressurises.