In this episode Tori visits the Severn Estuary and investigates a mysterious medieval ruin on a bleak island. Who built it, and why? Could it have been a religious site? Was it an early lighthouse or beacon? Further along the coast she visits the site of the medieval village of Sudbrook, which simply disappeared into the sea. She finds out how important the ferry crossing at Aust was during the 20th century and is shown a photo of a famous passenger who used it: Bob Dylan. And she investigates the story behind the very first attempt to build a tunnel underneath the Severn. At the moment, all this archaeology is available to anyone who cares to go looking for it, but rising sea levels and coastal erosion mean it won't be very long before it's all swept away. Britain at Low Tide captures a vanishing past for us all to see.

• Alex Langlands
• Tori Herridge