Steffan Powell explores five key factors - geology, wealth, people, war and innovation - to reveal the story of how the small town of Merthyr Tydfil became the heart of the Industrial Revolution. Beneath Merthyr's streets and hillsides today lies a vanished world - ironworks, foundries and entire communities lost to time. Now, using cutting-edge technology, that world is being uncovered. Drones, photogrammetry and AI reveal hidden tramroads, lost settlements and even forgotten voices from the past