Kids the world over, who enjoy making forts and digging in search of hidden treasures, might take inspiration from a Turkish man whose home renovations resulted in the discovery of an entire, ancient and subterranean city. Carved from the volcanic rock of the earth itself, the labyrinths, shafts and hidden entrances of Derinkuyu, in central Anatolia, were constructed with defence in mind. It's thought that as many as 20,000 people could have sheltered there, at depths of up to 60 metres: certainly, this was anything but child's play.