Bassenthwaite Lake is one of the largest lakes in the English Lake District. It is long and narrow, approximately 4 miles (6.4 km) long and ⁄4-mile (1.2 km) wide, but is also extremely shallow, with a maximum depth of about 70 ft (21 m).
It is the only body of water in the Lake District to use the word "lake" in its name, all the others being "waters" (for example, Derwent Water), "meres" (for example, Windermere) or "tarns" (for example, Dock Tarn). It is fed by, and drains into, the River Derwent. The lake lies at the foot of Skiddaw, near the town of Keswick. Some maps dating from the 18th century do in fact mark this lake with the name Bassenwater, and the use of name Broadwater for this lake is also attested.
The A66 dual carriageway runs roughly north/south along the western edge of the lake. With breathtaking views, the laybys are popular spots for photographers and bird watchers looking for osprey. The section running south towards Keswick was built on top of a former railway line leading to Penrith.