Marvel at the crystal caves and underground chapels of Poland's Wieliczka salt mine. Tour a secret Cold War bunker in England, unearth a 3,400-year-old royal settlement in Egypt, and try to solve the mystery of China's enigmatic Longyou Caves.

Crawl through a notorious brick-lined tunnel system in a historic Florida neighbourhood. Open a window into the lives of elite Greek families in Naples, explore a three-story underground city in Iran, and find 2nd-century barracks buried in Rome.