Will travels to North Wales to discover the forgotten history of the slate industry. Welsh slate once roofed the buildings of the British Empire and the industrial revolution. By the late 1800s, a third of all roofing slates on the planet came from North Wales. The industry produced communities, culture and an identity that are still alive today - but much of the history has been forgotten or vanished. Will tracks down some of the remains and relics from this vibrant period of industry by exploring deep inside the mountains of Snowdonia in some of biggest slate mines in the world.