It’s next to one of Italy’s most famous sites, but few visit this side

Ercolano station whips by, a blur of terracotta and graffitied concrete. We are supposed to be out there negotiating a bus ticket up Vesuvius, the only active volcano on the European mainland. Napoli’s low swagger demands local knowledge, which includes its public transport system of overlapping modes and companies. It’s a dialect we don’t speak, so we remain crammed among the armpits and steadying hands of tourists, squawking school children and their underpaid teachers.

At Pompeii we file out, running a gauntlet of hawkers. The next bus for Vesuvius leaves in 30 minutes, time to only glimpse the hollowed villas wrecked by the explosive eruption two millennia ago. The plan to cross from the volcano’s tourist-heavy front to the wild back country is entirely possible on maps, but nobody mentions achieving what seems entirely logical – catch a bus up and walk down. Any such gamble is made safer with more time and the sun is climbing.

The short bus zips up the incline and soon Napoli thins to villas in Vesuvius’s shadow, until all structures cease, replaced by balding scrub. The road of narrow switchbacks terminates at a carpark. Tourists file off yawning and stretching in the sulphurous alpine air. Below, Napoli crowds the gulf, glittering in the sun.

Tourists queue at the stall-lined ticket gates. “Is it possible to get through to the other side?” I ask the inspector. “Yes. I will radio ahead. Speak to Silvano.”

The ground is gravel. My partner Barbara, a geologist, bends to touch. “It’s hot,” she says. It has the warmth of a living being. The path climbs then surrounds the crater’s edge, the roped-off far side falling to a sheer drop. The audio app provides commentary as we climb. In 1944, occupying American military witnessed the last eruption, a rising column of black smoke for once unrelated to war. Both Shelley and Byron wrote poems about Vesuvius. The world’s fastest animal, the peregrine falcon, uses rising columns of warm air from the crater in its yearly migration.

At the crater’s edge, gawping tourists peer past the sharp lichen-encrusted rocks into the mouth: an immense pit of steaming gravel. Heat has melted gravel and ash into columns of solidified rock in the walls, Barbara explains. There are two types of volcanoes. Those in Hawaii are like a pot with no lid, where lava boils over and spills down the sides. The others are more like pressure cookers. Capped with rock, the pressure builds until the top explodes. Vesuvius began as the latter but the eruption that destroyed Pompeii blew off its lid, transforming it into the former.