Tim and Siddy reveal the incredible story of the Thames Tunnel – the first successful tunnel built under a major body of water anywhere in the world. It was built by Brunel and his father – and not originally intended for trains - but today links Rotherhithe and Wapping stations. Tim hears about the incredible engineering that made the tunnel possible – as well as the many failed attempts to get it right, including a major flood. Siddy walks the tracks in search of evidence of the tunnel’s original construction, revealing a series of striking arches.

In Lambeth North, Siddy explores this classic Leslie Green station, one of the least used in zone 1. But it has a significant history as a shelter during the Second World War, and as a training base for London Transport workers, especially women drafted in to work on the tube during the war.