Today's tour of Naples takes us around a new city, not the Naples we all know and love, but another more mysterious city, carved out beneath the surface. Naples has one of the most complex underground systems in the world thanks to the unique volcanic sandstone that characterises the landscape between Vesuvius and the Phlegraean Fields. This yellow durable sandstone or tufo was formed millions of years ago through geothermal and volcanic activity and was the ideal base for Naples' first habitants, some 5,000 years ago, to carve into grottoes and shelters. All this lurking just feet under the surface of today's vibrant and modern city.

From the Romans on
We thank the Romans engineering expertise for the complex system of water conduits, aqueducts and cisterns that runs beneath Naples and which was used up until relatively recent times to service the city. The sandstone caves were used for burial and religious purposes right through the early Christian period; the catacombs of San Gennaro, where Naples' patron saint lay until 817 and the burial chambers in the Sanità area are perhaps the best known. One of the most interesting underground walks starts from Vico Traetta ai Cristallini and weaves under an eighteenth century building. You can access the 'hidden city' from a courtyard off Vico Traetta where a series of steps lead you down to an underground temple decorated with sandstone pillars. You then enter narrow passageways through a complex network of frescoed burial. Another site worth seeing is the "cimitero delle fontanelle" (fountain cemetery) which can be reached from the parish of Santa Maria del Carmine. This underground graveyard was originally used as an illegal burial site, then as a paupers' graveyard and for victims of the plague and finally as a deposit for all bones unearthed from the city's overground cemeteries.

Into the heart of Naples
We continue our tour of Naples' fascinating underground system in one of the city's most fascinating areas - close to San Gregorio Armeno and the Dome. There you'll find the entrance to the Associazione Napoli Sotterranea (Underground Naples Association - tel. +39 081 296944). The association organises underground tours into the city's intricate sandstone bowels. First-timers will be surprised at the scale of the underground city - very different to the dank claustrophobic catacombs of Rome - and quite possibly disorientated by the silence of the subterranean system. The contrast with the chaotic, and at times riotous, city above could not be more stark, and, as you move down the shadowy steps your guide may ask you (ours did) to observe a minute's silence, - a mini endurance test of sensory deprivation.

Air raid shelters
The steps wind down under the city for around 40 metres, opening into a bell-shaped cave with passageways leading off it. One of the first things you notice are the electricity cables running along the roof of the cave, a reminder of how these cavities were home to hundreds of people during WW2 who used them as emergency air raid shelters. Their marks can still be seen in the carved benches, improvised divisions and temporary toilet facilities that remain to this day. The walls too bear witness to their enforced stay with graffiti including sketches of loved ones and portraits of D’Annunzio and Mussolini.
There's even a "wedding chamber", with the names of the betrothed and the date of the nuptials engraved on the wall. We hope they were left this space for the first hours of their marriage - but this, along with the answer to many other mysteries, we will never know.