The underground bases built by the German army during the occupation of Jersey, and a group of more than 20 underground cities in Turkey, one of which dates back to 800BC.

This episode explores the now hidden arches and vaults of South Bridge in Edinburgh, Scotland and reveal why this underground world has such a sinister past and how today it has been reimagined. Beneath Naples in Italy, under the city, viewers see how the excavation of the volcanic ‘tuff' to build above ground has left an extraordinary imprint underground. And finally, in Finland's capital Helsinki they've been developing an underground master plan for the entire city by reusing older subterranean spaces and creating new facilities for the population.