Overview: It's boom time as Wales becomes known the world over for one particular product - Welsh steam coal, the best you can get. In the space of 50 years, 'black gold' builds a new Wales. The coalfield pulls in hundreds of thousands of migrants with a different language and culture, becoming a bustling modern world of its own. Yet no sooner has Wales found itself at the centre of global trade, than the Depression causes an industrial crash with a bitter social fallout.