Ponder this – the ancient city of Derinkuyu in Turkey, estimated to have been constructed between the 5th and 10th centuries, is completely below ground. Virtually invisible from above, 11 stories deep, able to house at least 35 thousand people. It has an elaborate ventilation system, stables, kitchens, living quarters, and massive sliding stone doors, that make it impossible to penetrate. Quite an accomplishment for people with nothing more than hand tools.