Great County Adit

Overview

A single interconnected drainage system over 40 miles long, driven from 1748 to dewater more than 100 tin and copper mines across the Gwennap district -- one of the largest pieces of mining infrastructure ever built in Britain, and one that still flows today.

History

The Great County Adit was conceived by John Williams (born 1714), manager of Poldice mine at Scorrier, as a single shared drainage tunnel to spare individual mines the cost of pumping water out of their own workings independently. Driven from 1748, it reached Poldice by the late 1760s, was extended past Wheal Busy to Wheal Peevor by 1778, and gained a further westward branch, the Consols Adit, through the 1770s and 80s to drain the Consolidated and United Mines; a branch to Wheal Unity followed in 1792. Over time it grew into a single connected system of tunnels running more than 40 miles (64km) at an average depth of 80-100 metres, ultimately draining over 100 mines across the Gwennap mining district -- probably the most productive copper-mining area in the world during the 18th and 19th centuries. At its peak, in 1839, the adit discharged over 14.5 million gallons of water a day into the Carnon River.

Workings

The adit's portal lies in the Carnon Valley below the hamlet of Twelveheads, discharging via Carnon Valley into Restronguet Creek. By draining mine workings down to its own depth for free, it let mine owners avoid the cost of pumping engines for shallower ground, freeing up capital and steam power (contemporary accounts credit the surrounding district with more steam engines than the rest of continental Europe and America combined) for pumping and winding at greater depths beyond the adit's reach.

What remains

Although every mine it once served has long since closed and the tunnel system itself has had no maintenance since, the Great County Adit still flows: a 1980 measurement recorded a discharge of 500,000 gallons a day, a small fraction of its 1839 peak but a sign of just how much groundwater it still intercepts from the old workings above it.

Abandoned workings can be dangerous — unstable ground, uncapped shafts, flooding and bad air are common even where a site looks safe from the surface. This entry describes history and general location only; it is not an access guide.

Recorded claims

Individual factual claims, each attached to the source(s) that support it below. Where sources disagree, both are kept.

• The Great County Adit was conceived by John Williams, manager of Poldice mine, and driven from 1748, reaching Poldice by the late 1760s and Wheal Peevor by 1778, with the Consols Adit branch added through the 1770s-80s and a branch to Wheal Unity in 1792. ¹
• The completed system ran over 40 miles (64km) of tunnel at an average depth of 80-100 metres, draining more than 100 mines across the Gwennap district. ^1,2
• At its peak in 1839 the adit discharged over 14.5 million gallons of water a day into the Carnon River; by 1980, long after every mine it served had closed, it was still discharging around 500,000 gallons a day. ¹

Sources

1. (secondary) Great County Adit — Wikipedia
2. (secondary) The Great County Adit: Cornwall's marvel beneath our feet — Cornwall Heritage Trust

Record created: 29 August 2026 · Last researched: 29 August 2026