In Canada, almost two miles underground in the world's deepest copper and zinc mine, a mighty drill chews through rock, digging deep holes which are then filled with explosives and detonated, leaving behind debris packed with valuable minerals. Meanwhile in Helsinki, Finland, cameras follow a mission to excavate a new metro tunnel to improve commuter services - but working seven metres beneath the bustling city calls for precision engineering and one false move could lead to disaster. Plus, a mining machine works continuously underneath the Cheshire countryside to keep Britain's transport network moving. Last in the series.