A journey into some of the world's largest underground spaces, including a potash mine in Yorkshire, a rock cavern in Norway and an underground station in London.

First, we visit the deepest mine in western Europe. Boulby potash mine is situated on the north Yorkshire coast, approximately 1km deep into the Earth's crust and 8kms out into the North Sea.

Next, we travel to Norway where we see the world's latest rock cavern designed for public use. Gjovik cavern, a huge underground stadium, was used in the 1994 Winter Olympics and spans a huge 62 metres.

Finally, we take a look at the deepest excavation ever to take place in London - a hole built to encompass the Westminster underground station.

How Did They Build That? is a series that explores how new materials and construction techniques have allowed engineers, builders and architects to build some of the greatest man-made structures of all time.