An impressive underground network of tunnels and caves, curiously filled with vintage cars and forgotten World War II relics, lay forgotten under the ancient city of Naples.

In cities as old as Naples, residents have become used to unearthing classical Roman treasures, antiquated cisterns and other historic artefacts underneath their homes when it comes time to renovate.

More unexpected, however, was the accidental rediscovery in 2005 – when government geologists were checking the conditions of some quarries beneath the Monte di Dio neighbourhood – of an impressive underground network of ancient passageways, curiously filled with vintage cars and forgotten World War II relics, 150m below the Piazza del Plebiscito.

Constructed in 1853, the multilevel, 1,022sqm Bourbon tunnel network – today known as the Galleria Borbonica – was built by Ferdinand II of Bourbon, who, fearful of a revolution, needed a subterranean passageway large enough for troops and horses to travel from the Royal Palace to the military barracks.

Later, beginning in the 1930s, the tunnels were used as a warehouse for impounded and contraband vehicles since there was little available space in the tightly packed city. The passageways and cavernous chambers finally served as an air raid shelter during World War II, before being sealed off in the early 1950s to be forgotten forever.

But in 2012, Tonino Persico, a 90-year-old survivor who remembered hiding down here during the war, contacted Neapolitan geologist Gianluca Minin, who was leading the excavation of the rest of the tunnels, to alert him to the existence of a bomb shelter below the Palazzo Serra di Cassano, a palace behind the Piazza del Plebiscito.

It took three years for Minin and his team to clean the area ­of debris, but in December 2015, the Galleria Borbonica museum launched the deeply affecting Via delle Memorie tour that commemorates the lives of those who sought refuge here.

Paolo Sola, one of the founding members of the Galleria Borbonica museum, led me through the dimly lit tunnels. The stale air was impregnated with a malodorous blend of must, decay and, in places, even urine. As we walked on the hard-packed dirt, we passed small alcoves piled with battery-operated lanterns, shovels and other digging and surveying equipment – there was clearly still a lot of excavation work going on. Other times the labyrinth of tunnels would branch off into narrow stone corridors, which had once carried water from the palace’s cisterns.

Eventually we descended another stairway and came to the old air raid shelter. During the war, the stairways were unlit and, given the frenzy of the crowds, there was no guarantee one would get down safely.

One survivor, Professor Aldo De Gioia, who I’d previously met at Napoli Sotterranea, another underground museum and former air raid shelter, had told me about a tragic episode he still vividly recalled.

“I knew a girl, a beautiful girl called Edina, with brown hair and green eyes, who, in the rush to get to the shelter, was thrown down the stairs and trampled to death.”

De Gioia, now a spirited octogenarian, had been just eight years old at the time.

Sola asked if I wanted to listen to the air raid siren. I did. Hearing a siren in a film is one thing, but hearing that same haunting and very real sound in a bomb shelter made my chest tighten and filled me with a sense of dread.

“When enemy planes were sighted from Salerno and the islands of Ischia and Capri, the siren would be sounded three times,” he explained. “This meant that the planes were on their way and that residents had about 15 minutes to get down into the shelters. Over here is the phone that was used by the person responsible for communications,” he said, pointing to an old wind-up telephone hanging on a wall.

I noticed an old ampoule and rusted needle that had been found during excavations on display. According to Sola, many people had been wounded above ground or as they fled down the stairs to the shelter. Doctors tried to care for them the best they could in the small space that passed for an infirmary, but there were few medicines available.

Many would have also been ill and malnourished as the port and railroads had been bombed and the food that came in by truck went straight to the black market. The aqueduct had been blown up, so there was no clean water, and the electrical supply had been cut off; down below, the refugees were forced to rely on weak DC battery power.

Tragically, thousands of Neapolitans lost their homes to the bombs and needed to live in the shelters indefinitely. A dozen or so steps away from the infirmary were the toilets, separated from the showers and sinks by a cement partition. Adjoining this space was a large semi-enclosed area with three walls, where people slept. Now, a few thin mattresses were spread out for exposition purposes.

When researchers started excavating the tunnels, they found portable stoves, rusted tin pitchers and blackened pots, flattened doll carriages, mouldy furniture and other evidence of people trying to make some semblance of a life in the darkness and bone-chilling dampness. The poignancy of their suffering was palpable.

But life was not always bleak. While the women and men unable to fight shared their worry and grief for their husbands, fathers and brothers on the front, the children found ways to pass the time.

“We played games together,” said De Gioia. “We played hide and seek, but you could get lost down there.”

He remembered the barber who shaved men’s beards. “Normally he charged five lira, but when we were down in the shelter he would charge double!”

“Sometimes, someone would sing [the classical Neapolitan song] O Sole mio,” he remembered, wistfully.

In a soft, melodious voice, perfectly on key, De Gioia sang the first two verses of the chorus.

“Song is a good release for sorrow,” the survivor said.

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