He described the region stretching from Lincoln in the north to Cambridge in the south as a "hideous fen of a huge bigness".
At the end of the last Ice Age, it had been a well-drained, wooded plain. But as the glaciers receded and sea levels changed, its rivers choked and burst their banks. And then the rot set in.
Reeds and sedges grew in the waterlogged wilderness, turning to peat as they decayed. After 8,000 years, this fibrous, black sludge was several metres deep. And then somebody had a bright idea. Why not drain the disease-ridden swamp, and turn it into farmland? Think of the money to be made.
It was clearly a job for a Dutchman, so in 1630, a bunch of wealthy adventurers hired an engineer called Cornelius Vermuyden, who dug the first of many artificial rivers that would speed the flow of water into the Wash.
And that's when the problems began. No sooner had they begun to dry out than the new fields of richly fertile peat began to sink. Before long, the land was below sea level, and the only way to keep the new rivers flowing downhill was to bank them up. This meant that water from the fields had to be lifted over the banks by wind pumps.
Exposed to oxygen for the first time in several millennia, the peat now began to waste away, turning to dust that blew in the winds that swept across the flat, black fields. It was as if the engineers were locked into an unwinnable race against the forces of nature. For as windpower was supplanted by steam and then diesel, the shrinkage and wastage accelerated, which in turn meant higher river banks and an ever greater risk of flooding.
Where will it end? Nobody knows for sure. But some conservationists, mourning the loss of Europe's wetland habitats, would welcome a return of Guthlac's "hideous fen". And with sea levels rising as never before, they might just get their way.