Great County Adit

Construction started in 1748 and it eventually reached a length of over 40 miles (64 km) of a tunnel, providing drainage to over 100 mines at an average depth of 80–100 metres (260–330 ft).

[1][2] The adit was the brainchild of John Williams (born 1714) of Scorrier who was the manager of Poldice mine.

[1] The Carnon River empties into Restronguet Creek (a tidal arm of the Carrick Roads upstream from Falmouth).

Major floods in the winter of 1876 caused large quantities of gangue and silt to be washed into the higher part of Restronguet Creek, permanently damaging navigational access to the upper quays at Devoran.

[4][5] Although all the mines served by the Great County Adit have closed and it is unmaintained, it still drains many of their underground workings today; in the summer of 1980 the flow was 500,000 gallons per day.