Dorothea quarry is a disused slate quarry in the Nantlle Valley area in North Wales. It covers a large area near the village of Talysarn and contains three flooded deep lakes.

The quarry commenced working in the early 1820s, though there were a number of smaller workings on or near the site before this. About 1829 it was leased by William Turner who named the workings Cloddfa Turner. In the 1830s the quarry was generating £2000 profits per year (equivalent to £186,792 in 2016). Turner's son took over as manager and renamed the quarry Dorothea, apparently after the wife of the landowner Richard Garnons. Profits began to fall in the 1840s, and in April 1848 the quarry was put up for sale, with 22 years remaining on the lease.

It was the largest quarry in the area, employing 200 men and producing 5,000-6,000 tons of finished slate a year. A group of quarrymen led by John Robinson, William Owen and John Jones purchased Dorothea from Turner for £3,000 - equivalent to £325,276 in 2016. They renewed the lease of the quarry in June 1851. The quarry was put up for sale again in 1864, at which time production was given at 1,000 tons per month. No buyer was found, instead John Williams of Denbighshire gradually bought out many of the existing shareholders, and by 1879 he had amassed more than 70% of the shares.

Production peaked in 1872 at 17,442 tons, though 1875 was the most profitable year, generating £14,738 (equivalent to £1,433,233 in 2016). In the 1930s over 350 men were employed at Dorothea. Production dropped significantly after the start of World War II and the quarry closed in 1970.

Since quarrying ended in 1970, the Dorothea quarry has flooded and become a popular site for scuba diving (even though there are no facilities provided, and diving is officially banned in the quarry); the unregulated nature and depth of the site has encouraged some divers to overestimate their capabilities – in the decade 1994–2004, 21 divers lost their lives in the quarry.

The quarry sits at the bottom of the wide Nantlle valley and consists of six pits, the deepest dropping 106m from the surface. The slate veins here run vertically, allowing unusually deep vertical pits to be dug. Because the pits fall below the water table they needed to be constantly pumped to stay dry. A Cornish beam engine was installed in 1904 to pump the pits; it stayed in use until 1951 when it was replaced with electric pumps. This was the last new Cornish engine to be built. It remains in situ in its Grade I listed engine house.

The quarry was one of the first users of the 3 ft 6 in (1,067 mm) Nantlle Railway in 1828. It later developed an extensive internal tramway system of 2 ft (610 mm) gauge. Dorothea was one of the first users of De Winton locomotives in 1869.

2 ft (610 mm) gauge locomotives known to have worked at Dorothea.

Name	Builder	Works No.	Date built	Type	Notes
	De Winton		c1869	0-4-0VB	Scrapped by 1914
Glyn	De Winton		1874	0-4-0VB	Scrapped by 1914
Dorothea	Hunslet	763	1901	0-4-0ST	Worked until 1942. Now preserved on the Launceston Steam Railway
Wendy	W.G. Bagnall	2091	1919	0-4-0ST	Purchased from the Votty & Bowydd Quarry in Blaenau Ffestiniog in 1930. Preserved by the Hampshire Narrow Gauge Railway Trust and donated by them to the Statfold Barn Railway
	Deutz		1926	4wPM	Purchased from the Votty & Bowydd quarry in March 1930. Believed scrapped.
	Lister	33527		4wPM	Out of use in October 1960. Scrapped in 1964.
	Lister	3916		4wPM	ex-Tarmac Limited contract in Wolverhampton. Out of use by 1964.
	Lister	3950		4wPM	ex-Tarmac Limited contract in Wolverhampton. Out of use by October 1960. Scrapped in 1964.