The tower is constructed not of the typical steel but of so-called "puddle iron," a popular iron-treatment of the 19th century that involves heating and folding over sheets of iron. It performs totally differently than steel, and so the engineers had to pretty much start from scratch to build an accurate model—they called in materials engineers to reconstruct the long-forgotten puddle iron and perform various tests on it. ... As it turns out, the tower's strength is largely due to that near-forgotten puddle iron—the engineers estimate that the tower will still be standing for at least another two or three centuries.