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Iron and manganese are the two key ingredients that enabled the mass production of steel - one of the most versatile and complex materials known to humanity. Justin Rowlatt chews on salad leaves with Andrea Sella of University College London, who explains how manganese is present in all plants and plays a key part in photosynthesis and ultimately oxygen production.

He also travels to Sheffield to visit a modern steelworks - the specialist engineering steel-maker Forgemasters - where Peter Birtles and Mark Tomlinson give a taste of just how hard it is to produce unbreakable parts for nuclear power stations and oil rigs.