Justin Rowlatt explores two moments in history that transformed this most abundant of metal elements into the key material out of which modern life is constructed.

In the first of three programmes, Justin travels to St Paul's Cathedral, where professor Andrea Sella of University College London recounts why Christopher Wren was so vexed that the new railings were built out of cast iron. Then onto Ironbridge, where curator John Challen tells how the world's first major iron structure came into being. And, Justin ends at Cyfarthfa in Wales, once home to the world's biggest ironworks, where historian Chris Evans explains why puddling and rolling are far more world-changing than they sound.