Rail historian, Tim Dunn and Siddy Holloway from the London Transport Museum, 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. Without the Thames Tunnel, the London Underground as we know it wouldn't exist. 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.At the London Transport Museum depot, Tim gets the inside story on plans to extend the Bakerloo line into south east London - including the building of a brand new 21st century tunnel.