It is easy to forget that the UK was once a great source of metal, trading tin with the Romans (or so I am told) before the invasion 2,000 years ago.
Originally dug from surface veins, these metal mines were deepened until the ore ran out or water was reached, after which pumping technology limited operations.
An energy-free alterative to pumping – once the blasting was over – were long gently-sloping drains that allowed water to flow out to the surface under gravity.
And once such a drain was dug, pumps could discharge into them from even greater levels.
Two amazing drains are 8km-long Great County Adit in Cornwall, and the 16km-long Milwr Tunnel in North Wales.
The Adit was started in the mid 18th century and, by the time it was no longer used in the late 19th century, had some 60km of branching tunnels behind its mouth at Twelveheads.
According to this site the longest branch is just under 9km and the whole thing drained 100ish mines covering 31km2.
The Milwr Tunnel is much younger – started after the Adit was abandoned late in the 1800s, and in use until the 1980s.
Intended to drain lead mines, its building revealed further lead deposits.
While the Adit drained into a stream in-land, to get the maximum possible drop, the Tunnel exits on the coast beneath the high-tide line and was closed by automatic gates.
Both of these huge undertakings still discharge millions of litres into the open every year, and have been responsible for some odd goings on.
For example, flooding from the Great County Adit deposited so much debris from the inside of Cornwall into Restronguet Creek that the coastal town of Devoran ceased to be a port.
And a few decades later, during the First World War, extending the Milwr Tunnel turned of the holy spring at Holywell.
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