22/10/2026
Dorothea Quarry is a historic, disused slate quarry located in the Nantlle Valley near Talysarn, Gwynedd, North Wales.
Key Milestones in Dorothea's History
• Beginnings (1820s): Small-scale slate workings began in the early 1820s. In 1829, William Turner leased the site and named it Cloddfa Turner.
• Renamed Dorothea (1830s): The quarry was renamed Dorothea, likely in honor of Dorothea Garnons, the wife of local landowner Richard Garnons.
• Worker Ownership and Growth (1840s–1850s): After financial struggles, a group of local quarrymen bought the operation in 1848, and by 1848 it employed about 200 men. Later, John Williams of Denbighshire acquired a controlling share by 1879.
• Peak Production (1870s): Slate output reached its height in 1872, producing 17,442 tons. The quarry was heavily linked to the local transport network via the 3 ft 6 in Nantlle Railway (opened 1828) and internal narrow-gauge tramways.
• The Pumping Challenge: Because the slate veins ran vertically, miners dug six massive pits reaching down to 106 meters (348 feet), well below the natural water table. To combat constant flooding, a massive Cornish beam engine was installed in 1904. This engine—housed in a Grade I listed building—operated until 1951, when electric pumps took over.
• Decline and Closure (1970): Production dwindled following World War II, and the Dorothea Slate Quarry Co. Ltd entered liquidation, leading to the quarry's permanent closure in 1970.
After Closure and Modern Status
• Flooded Pits: When the pumps stopped, the deep pits filled with rainwater, turning into striking turquoise-blue lakes surrounded by dramatic slate cliffs and moss-covered ruins.
• Safety and Diving: The submerged pits—some over 100 meters deep—became a controversial and hazardous unofficial scuba-diving site. Between 1994 and 2004, 21 divers lost their lives in the cold, debris-filled depths, leading to strict local warnings and restricted access.
• Preservation: The site is now recognized for its industrial archaeology, featuring the iconic 1904 Cornish beam engine house and remnants of the old settlement.
