The tunnels remained disused until the beginning of the 1930s, when they became a warehouse for impounded and contraband vehicles. During World War II, the subterranean space was converted into a military hospital and a bomb shelter. After the war, the tunnels became a dumping ground of wartime rubble including building debris, old televisions sets and refrigerators, destroyed cars and motorcycles, and pro-fascists marble statues before it was sealed up and forgotten.
Today, these tunnels with their accumulated debris have been turned into a gallery known as Galleria Borbonica, where the public can see interesting displays of vintage cars and motorbikes, old shelter spots, ancient cisterns and much more.
http://www.amusingplanet.com/2016/10/the-forgotten-tunnel-under-naples.html