Three hundred years ago, Abraham Darby fired up his furnace in Coalbrookdale, Shropshire. It was the spark that lit the Industrial Revolution. Darby's technological leap would allow iron to be manufactured on an enormous scale, opening the door to the creation of the machines, factories, trains and ships that made the modern world. Helen Mark visits the original foundry, joins in an archaeological dig on the site, hears how the Aga company keeps the gorge's traditions alive and meets a contemporary artist inspired by the tough life of the original ironworkers.