The term "underground Naples" indicates the dense and complex network of tunnels and cavities, which are found in the Neapolitan subsoil and which form a real city that follows, in negative, the surface city. The underground city extends under the entire historical centre, linked to it myths and legends still alive today in the collective imagination of the Neapolitans. The galleries underlying Naples have been used in various ways over the centuries. Born following the extraction of tuff for the construction of the city, they were then used as an aqueduct and as a refuge during the Second World War.