THORNTON, ENGLAND—Iron was smelted in northern England some 2,400 years ago, The Yorkshire Post reports. An investigation conducted ahead of a construction project unearthed the bottom of a large furnace consisting of a pit that would have been packed with green wood. In this system, molten waste slag dripped down into the pit, slowly burned away the green wood, and collected as it cooled. The iron ore that was smelted at the site was likely col…