A secret underground passageway commissioned by King Ferdinand II of Bourbon in 1853, originally intended as an escape route from the Palazzo Reale to the barracks. The tunnel was later used as a WWII air-raid shelter and civil defense depot; it now holds an eerie collection of abandoned vehicles, wartime artifacts, and civil defense equipment.
Several tour formats available — the standard walking tour, a more theatrical "adventure" route, and a boat tour of the flooded sections. Book online; tours run in English and Italian.