Naples sits on volcanic monsters - and one of them threatens to consume the city

Beneath one of Italy's most densely populated cities, there is danger brewing. Naples is a city that moves at more than one pace. At the fastest speed, you have the chaos of the day-to-day: cars and motorbikes hurtling through the Italian city's streets like blood cells within a vein. Then there is a slower evolution, that of human history. This is a metropolis still shaped by decisions made hundreds of years ago: networks of narrow alleyways that no city-planner would ever design today; modern offices and hotels pressed against Roman ruins and centuries-old villas.

What's less obvious is that Naples also has a geological pace – and it's this one that holds the most power. Mostly it runs at a barely perceptible creep, but every so often, it accelerates into catastrophe: an eruption of fire and rock that disrupts all other timescales in the city. The volcanoes of Naples keep a different time to human beings – evolving over decades, centuries or even millennia – so it can be easy to assume they are static. This is far from the truth.

Vesuvius is the best-known, most famous for destroying Pompeii and Herculaneum in 79AD. It towers above the city. But there are other subterranean threats here, and some of them have far more bottled force. In fact, one of these colossal volcanoes is showing worrying signs that it is about to erupt – and there are hundreds of thousands of people living right on top of it.To find out what it's like to live and work alongside these monsters, I recently visited Naples to make a film for the BBC, speaking to the geologists who study and watch them 24-hours a day. I discovered a city creeping towards disaster, but no-one can say for sure when or where it will happen.One of the last times Naples experienced a major volcanic event was exactly 80 years ago this week. 

In mid-March 1944, during World War Two, caches of magma and gas that had been slowly building beneath Vesuvius suddenly began to erupt. A US military doctor, Leander Powers, who was stationed in Italy at the time, described what he saw: "While we were just finishing supper, someone called to say there were huge red streams of lava flowing down the sides of Mount Vesuvius…we could see a glow in the sky. All during the night and Sunday there were quakes of the earth with tremendous roars – similar to thunder... The windows rattled, and the entire building vibrated."