|Location||Snowdonia, North Wales|
|Type||natural lake, reservoir|
|Primary outflows||Afon Dulyn|
|Basin countries||United Kingdom|
|Surface area||33 acres (13 ha)|
|Max. depth||189 ft (58 m)|
The lake covers an area of only 33 acres (130,000 m2), yet is extremely deep - it reaches to a depth of 189 feet (58 metres). Less than a kilometre to its south lies the smaller Llyn Melynllyn.
Cliffs rise steeply from the lake edge up to the summits of Garnedd Uchaf and Foel Grach, giving it a dark brooding appearance - hence its name. A number of superstitions are connected with the cwm, which has also been the site of a number of small aeroplane crashes. 
The lake has a dam, originally built in 1881, to increase its water capacity, but it has been altered several times since. As a reservoir Dulyn (along with Melynllyn) provides water for the town of Llandudno. The pipes cross the River Conwy at Tal-y-Cafn bridge.
- The Lakes of Eryri by Geraint Roberts, Gwasg Carreg Gwalch, 1985
- "The Lakes of North Wales" by Jonah Jones, Whittet Books Ltd, 1987
- "The Lakes of Eryri" by Geraint Roberts, Gwasg Carreg Gwalch, 1985
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