Underground cities have long been sci-fi fodder, but now governments and planners are taking them seriously. One of the biggest challenges to overcome is convincing people to be comfortable underground

Back in 1800 BC, the people of the Cappadocia region of modern-day Turkey decided their environment was so hostile – with extreme weather and the constant threat of war – that they dug an entire city underground. Derinkuyu, the oldest underground city still in existence, housed 20,000 people, providing schools, houses, shopping areas and places of worship protected by large stone doors which allowed each floor to be closed off separately.