Puffing out clouds of vapour continuously is a volcanic phenomenon not far from Naples. And it's not Vesuvius, whose spectacular eruption doomed the flourishing Roman cities of Pompeii and Herculaneum in the year 79.

It's the Phlegraean Fields - Campi Flegrei in Italian - a sprawling constellation of ancient volcanic centres near the Tyrrhenian Sea and extending under the sapphire-blue waters. The Solfatara Crater, a steaming, smelly reminder of nature's explosive power, is the star of the bunch. An eruption in the area some 37,000 years ago spread ash as far as Siberia, and another huge blast left its mark about 15,000 years ago. “Phlegraean” means “burning,” and the fields are scorching hot only a few inches below the surface. Little wonder the ancients believed the caldera was the door to hell. These days, Italian schoolchildren and tourists from around the world roam across Solfatara's flat and dusty crater bed, a few miles from the pleasant port town of Pozzuoli.

White clouds of steam pour out of a largely barren hillside, the inner slope of what was left from volcanic eruption. Another attraction is a bubbling, gloppy-looking muddy pond, like some giant chocolate pudding boiling over on a stovetop. The main fumarole is nicknamed “Big Mouth.” The only organisms that seem to thrive in the high temperature immediately around it are algae, which give the area a green tinge. Geologists monitor the volcano by checking temperatures, chemically analyzing gases emerging from fumaroles and scrutinizing the ground to see if it is rising or falling. The last notable eruptions in the Phlegraean Fields occurred in 1158 at Solfatara and the other at what is now another volcanic cone, Monte Nuovo (New Mountain) in 1538.

But the terrain is ever evolving. A few years ago, scientists, using satellites and GPS instrumentation, concluded that the Phlegraean Fields area had risen by some 30 centimetres (12 inches) over a decade. In the early 1980s, thousands of residents of a Pozzuoli neighbourhood were evacuated when the ground rose up by some 1.8 metres (6 feet) and earthquakes rocked the area. Solfatara is safe to visit. Just don't step too close to the fumaroles to shoot a selfie.

And beware the steaming hot brick structures leftover from a popular spa attraction in the late 1800s. The patrons of that era believed hanging out in the super-hot natural “sweat rooms” was good for what ailed them. Known as “stufe,” (stoves), the sweat rooms were created by excavating ancient grottoes in the side of the volcanic mountain. The pair of sweat rooms was named “Purgatory” and “Hell.” These days, the visitor who thrusts a hand inside and holds it a few inches below the steaming brick abruptly yanks it back because of the intensive heat. One recent, brilliantly sunny spring day, the air temperature measured 20 C (68 F) but a mere few inches under the surface, the temperature was a sizzling 78 C (198 F) degrees. That hints at powerful heat of a magma chamber some 3 to 4 kilometres (nearly 2 to 2.6 miles) deep in the belly of the beast.

Many combine an excursion to Solfatara with a stop in Pozzuoli, whose remarkably well-preserved ancient Roman marketplace is now the centerpiece of a lovely town square near the sea. Geologists point out a link between the town and the Phlegraean Fields. Holes bored into the marketplaces' columns by sea mollusks indicate the changes in ground level in respect to sea level throughout history. From Pozzuoli's port, you can hop a boat to Ischia island, which abounds in spas advertising thermal waters, a most delightful legacy of the volcanic area.