In this episode the team travel to Rotherhithe for a very rare opportunity to walk through the Thames Tunnel, whilst London Overground had stopped trains for scheduled upgrade works. Chris demonstrates how a tunnelling shield was key to the success of building this as the first underwater tunnel in the world in 1843. Siddy uncovers stories about the Brunnel dynasty who masterminded this engineering feat. Laura explains who we turned this into a Hidden London public tour and Alex turns shop keeper for one morning only.