Dorothea quarry is a disused slate quarry near the village of Talysarn and contains three flooded lakes.

The quarry was leased by William Turner in 1829 who named the workings Cloddfa Turner (Turner Quarry), but it was his son who renamed the quarry ‘Dorothea’, apparently after the wife of the landowner Richard Garnons. 

It was the largest quarry in the area, employing 200 men and producing 5,000-6,000 tons of finished slate a year. Slate production peaked in 1872 at 17,442 tons but dropped significantly after the start of the Second World War and the quarry closed in 1970.

Since then, it has become a popular site for Scuba Divers (even though diving is officially banned in the quarry). The slate veins run vertically, which allowed unusually deep vertical pits. Unfortunately, this has encouraged some divers to overestimate their capabilities and in the decade 1994–2004, 21 divers lost their lives in the quarry.