Turkey's Cappadocia region has many awe-inspiring, ancient underground cities, but rambling and multi-chambered Derinkuyu is the largest. Dating back to the 7th and 8th centuries BC, the sprawling subterranean commune was built to protect its inhabitants from invading forces during sieges. As well as being capable of housing between 20,000 and 50,000 people, the network of man-made caves also housed food stores, stables, churches and wineries. Various companies offer tours of this subterranean world – check with individual operators like Turkish Heritage Travel for availability.