In the early hours of 13 February 1322, the central tower of Ely Cathedral collapsed with a sound so loud the monks thought there had been an earthquake.
Rather than rebuild it as it was, the monk in charge, Alan of Walsingham, designed something that had never been attempted before: an octagonal tower topped with a wooden lantern, too wide for stone to span.
It took 18 years to build and still stands today, one of the great engineering feats of medieval England.