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Dallas Campbell reveals a fascinating and secret world hidden below Britain. In this episode he explores how what goes on underground keeps our country on the move. He delves into the past to discover how a secret wartime pipeline is now delivering fuel to Heathrow Airport. Extraordinary computer graphics lay bare the underwater engineering genius that allowed the iconic Forth Rail Bridge to be built in the 19th century.

Along the way Dallas meets some of the hidden army of workers that keep Britain running from underground, from the drivers of the largest tunnelling machines in the world to the engineers running a vast power station under a mountain in Wales. In one memorable scene, he helps dislodge a 'fatberg' that's blocking one of London's sewers. And he does some secret filming of badgers that are threatening the foundations of a primary school and helps to relocate the whole sett.