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It's being built on the Middle Level Drain at St Germans, near King's Lynn, to protect half the Fens from rising sea levels and increasingly-frequent storm surges.
When it becomes operational next February, it will be able to shift 1000 tonnes of water a minute.
The Middle Level Drain is fed by a system of drains stretching miles inland, including the Sixteen Foot, Forty Foot, Old Nene and Popham's Eau.
It protects tens of thousands of homes and thousands of hectares of low-lying farmland, much of it below sea level, from flooding.
When the drainers reclaimed the Fens, they created mile upon mile of waterways which centuries later have become a Mecca for pike anglers.
But they didn't realise the peaty soil would shrink as it dried out. And parts of the area are now 20ft below mean high tide levels.