Tunnel Quarry, Corsham

Overview

The largest of the Corsham Bath stone quarries, opened in 1844 beside Brunel's Box Tunnel and raising a hundred thousand tons a year by 1864. The War Office bought its fifty acres in 1935 and turned it into the country's most completely fitted underground ammunition depot, with its own railway platforms, conveyors and power station.

History

Tunnel Quarry - also called Corsham Down Quarry, or No. 7 Quarry - owes its existence to a railway. When Brunel's Box Tunnel was driven through the hill between Bath and Chippenham and completed in 1841, the excavations showed that vast quantities of unworked freestone lay under the Corsham plateau, and the quarry industry in the district expanded rapidly from that date. Work started at Tunnel Quarry in 1844, and it became the original major quarry of the area: a broad-gauge railway ran into the underground workings, and by 1864 it was producing a hundred thousand tons of stone a year. Bath stone here was mined rather than quarried in the open. The stone's consistency lets the roof span a considerable distance unsupported, so the removal of stone leaves large galleries and tunnels behind it - which is exactly why, twice in the twentieth century, the War Office came looking. By 1934 the government had concluded that another European war was coming and that the ammunition reserves had to be moved beyond the reach of German bombers. The Royal Engineers investigated underground sites across the country and settled on the Wiltshire limestone quarries as the best of them. The Bath and Portland Stone Company were approached, survey work was done in January, and the fifty-acre Tunnel Quarry was bought on 15 August 1935. More quarries followed by 1937 as the demand for anti-aircraft ammunition grew, and together they became Central Ammunition Depot Corsham: Tunnel Quarry itself, Eastlays at Gastard, Ridge Quarry and Monkton Farleigh. Conversion was a vast undertaking. Between twelve and twenty-five thousand unskilled labourers under the Royal Engineers cleared some two million tonnes of waste stone over four years, straightened irregular workings into roads and strengthened the roofs. The storage environment was the harder problem: the quarries hold a fairly constant fifty degrees Fahrenheit but the humidity is very high, which rules out all but the shortest storage of most materials, so air conditioning had to be built in. What the depot was given is the reason it is now scheduled: two underground railway platforms served from the Box Tunnel itself, a conveyor control room and conveyors, a slope shaft, a locomotive shed and an underground power station. It is the most extensive and most fully fitted-out example of an underground ammunition depot in the country, one of only thirteen quarries requisitioned or bought for the purpose in the 1930s and 1940s. The adjoining Spring Quarry was afterwards converted, at a cost of millions, into the alternative seat of government for a nuclear war.

Geology

Worked the Bath stone freestone of the Corsham plateau, a Jurassic oolitic limestone whose consistency allows a wide unsupported roof span, which is what leaves the broad galleries and tunnels behind the working.

Workings

Underground stone mining rather than open quarrying, with a broad-gauge railway carried into the workings. After 1935 the galleries were regularised into roads and the roofs strengthened, and the depot was fitted with two railway platforms reached from the Box Tunnel, conveyors and a conveyor control room, a slope shaft, a locomotive shed, an underground power station and air conditioning against the high natural humidity.

Closure

Stone working gave way to military use. The fifty-acre quarry was bought by the War Office on 15 August 1935 and converted into the principal depot of Central Ammunition Depot Corsham, the role it and its neighbours were kept for long after the war.

What remains

In defence use and scheduled as a monument. The site remains in the Ministry of Defence estate at Corsham, and the underground infrastructure of the ammunition depot survives substantially complete.

The depot's underground infrastructure is the survival that matters: the railway platforms, conveyor control room, conveyors, slope shaft, locomotive shed and power station, which are what the scheduling covers.

Related mines

• Monkton Farleigh Quarry — Both were Bath stone workings taken for Central Ammunition Depot Corsham in the rearmament of the 1930s; Tunnel Quarry was the principal depot and Monkton Farleigh a sub-depot.

Nearby mines

• Monks Park Mine — 2.6 km SE
• Box Quarry — 3.9 km W
• Monkton Farleigh Quarry — 6.2 km SW
• Combe Down Stone Mines — 12.4 km SW, Somerset
• Seend Ironstone Quarry — 12.4 km SE

Straight-line distance, within 15 km. Show on the map.

Recorded claims

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

• The completion of Brunel's Box Tunnel in 1841 revealed vast quantities of unworked freestone and the Corsham quarry industry expanded rapidly from that date. Work started at Tunnel Quarry in 1844; it was the original major quarry of the area, had a broad-gauge railway running into the underground workings, and by 1864 was producing a hundred thousand tons of stone a year. ¹
• The Bath and Portland Stone Company were approached by the War Office and, after survey work in January, the fifty-acre Tunnel Quarry was purchased on 15 August 1935. Further quarries were bought by 1937 as the need for anti-aircraft ammunition grew, and together they became Central Ammunition Depot Corsham. ^2,3
• The Wiltshire stone mines hold a fairly constant temperature of about fifty degrees Fahrenheit, but their very high humidity precludes all but very short-term storage of most materials, so the conversion had to provide air conditioning as well as roads and roof strengthening. ²
• Tunnel Quarry is scheduled as the most extensive and fully fitted-out example of an underground ammunition depot, one of only thirteen quarries requisitioned or bought for the purpose in the 1930s and 1940s. Its underground infrastructure includes two railway platforms, a conveyor control room, conveyors, a slope shaft, a power station and a locomotive shed. ^2,4

Photographs & further reading

Sources

1. (secondary) Chronological History of Bath Stone Quarrying — Cerberus Spelaeological Society Journal
2. (secondary) Corsham Ammunition Depot and Standby Government Headquarters — Subterranea Britannica
3. (secondary) Corsham Computer Centre — Wikipedia
4. (primary) MoD Corsham: Tunnel Quarry, Corsham (NHLE 1409857) — Historic England

Record created: 22 September 2026 · Last researched: 23 September 2026