The Bourbon Tunnel, Naples, Italy

Conceived of by Ferdinand II in 1853 as an escape route from the Royal Palace that was never completed, the Bourbon Tunnel is part of the Carmignano Aqueduct system. Built in the early 17th century, the aqueduct supplied water to the Monte di Dio district until 1866. During WWII, this cavernous structure served as an air raid shelter and military hospital, providing aid and protection to some 10,000 Neapolitans. After the war, the tunnel was used an impound lot until the 1970s.