Will discovers the forgotten story of how Wales once led the world in copper production by visiting some extraordinary and abandoned locations. He begins his journey in Anglesey on Parys Mountain, once the world’s biggest copper mine. Will goes beneath its Mars-like landscape into the miles of tunnels that were first constructed in the 18th century. Then in south Wales, Will unearths the remains of Swansea’s once huge copper industry, tracking down the final relics of the lost industrial world of Copperopolis. The episode concludes with Will cave-diving through a 19th-century flooded mine to discover its important role in Wales’s industrial revolution.