Llyn Cowlyd is the deepest lake in north Wales. It lies in the Snowdonia National Park on the edge of the Carneddau range of mountains, at a height of 1,164 feet (355 m) above sea level. The lake is long and narrow, measuring nearly 2 miles (3 km) long and about a third of a mile (500 m) wide, and covers an area of 269 acres (1.1 km2). It has a mean depth of 109 feet (33 m) and at its deepest has given soundings of 229 feet (70 m), this being some 45 ft (14 m) greater than its natural depth, the water surface having been raised twice by the building of dams.
The surrounding hills drop steeply to the water's edge, from Cefn Cyfarwydd and Creigiau Gleision to the east, and Pen Llithrig y Wrach to the west, and as a consequence have not been forested in the 20th century, as were the slopes of neighbouring Llyn Crafnant. Indeed there is not a tree to be seen, and the general aspect is one of bleakness. Dependent on the weather conditions, the waters often appear dark.