Just over 1216 years after the birth of Christ (according to Dionysius Exiguus), the part of East Anglia known as The Wash was sitting there being marshy. It still is. The only difference was that back in October of that year, King John sent his crown jewels across this particular piece of marshland. He was fleeing from Prince Louis of France, whom the barons had decided would be a fitter ruler of English after King John reneged on the signing of the Magna Carta, and was trying to cross from King's Lynn to Newark. He stayed in Wisbech overnight whilst he sent his baggage train onwards across The Wash. Unfortunately, a deluge from the River Nene, and sharply rising tides, engulfed the baggage train with the subsequent loss of the treasure.
There is a rumour that Robert, 3rd Lord Tiptoft (or Tibetot, or Tybotot) recovered the trove in the late 1300s, which led to the unexplained rise in his personal wealth. It's also been suggested that the original legend may be just a fabrication, or that it was arranged for the jewels to be secreted off. The legend is, however, widely believed, and plenty of people have looked for the treasure over the years. It'd currently lie 20ft or so beneath reclaimed farmland in Lincolnshire, between Walpole Marsh and Foul Anchor near Sutton Bridge, if you want to go and look for it. The OS map reference is TF 473 177 GB.
Cite: Palmer, S.B. (2005). "The Wash", in: What Planet is This?
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