The Great County Adit is a system of underground tunnels that drained tin and copper mines between Redruth and Bissoe in west Cornwall, England. It was begun by William Lemon, a major adventurer[a]Shareholder in the mines, to drain the Poldice Mine, which produced tin and copper. Mining there had begun before 1512, but by the 1730s its two water-powered pumping engines could not keep the workings dry. Lemon decided that an adit from Wheal Nangile near Twelveheads to the the Poldice sett[b]The area from which the company could extract minerals would dewater the mines by gravity into the Carnon River. Work on the adit, carried out using hand tools and gunpowder, was supervised by the mine captain[c]Manager, John Williams. By 1756 it was 1.5 miles (2.4 km) long and had reached the eastern edge of the Poldice sett boundary, but it was extended over time with branches to other mines. At 100 fathoms (183 m), Poldice was the deepest and largest mine in the area, and water flowing into it increased as more mines were opened. By the mid-1770s the enlarged network of adits was referred to as the Great County Adit.
The completed adit drained an area of about 12 square miles (31 km2), containing 100 mines in five parishes. More than 40 miles (64 km) in length, its longest branch was 3.4 miles (5.5 km). The adit drained mines to a depth of 40–60 fathoms (80–120 yards). In 1817–1819, more than 11 million gallons of water flowed from the adit every day. At its peak in 1839, when more steam engines were pumping water into the adit than were used in all of continental Europe and America combined, 14.5 million gallons a day flowed into the Carnon River.
Such was the adit’s importance, that in May 1843, the Mining Journal described it as “the most extensive, valuable, and systematic undertaking of the kind in Cornwall – perhaps in England, and we believe, but few in the world exceed it in importance”. It was maintained by a management committee of mineral lords and mine captains until 1886. Maintenance took place during the summer, when flows were lower.
Over time, the adit had cost about £250,000 but it saved up to £20,000 annually on pumping costs. More than thirty steam engines pumped water into the adit in the 1860s, but as the mining industry declined, by 1875 many mines drained by the adit had ceased production.
In winter 1876 poor weather caused flooding in the adit and torrents of water, mining debris and silt were expelled into the Carnon River and deposited downstream at Devoran. Just ten mines along the adit were working after 1870, and by 1900 only Wheal Peevor, Killifreth Mine and Wheal Busy were still open. Activity continued intermittently in the 20th century at a few mines, none of which are now operational. The adit is untended, and by 1980 its outflow had dropped to half a million million gallons per day.