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Wast Water or Wastwater is a lake located in Wasdale, a valley in the western part of the Lake District National Park, England. The lake is almost 3 miles (4.8 km) long and more than one-third mile (540 m) wide. It is a glacial lake, formed in a what is a fine example of a glacially 'over-deepened' valley. It is the deepest lake in England[2] at 258 feet (79 m). The surface of the lake is about 200 feet above sea level, while its bottom is over 50 feet below sea level. It is owned by the National Trust. The head of the Wasdale Valley is surrounded by some of the highest mountains in England, including Scafell Pike, Great Gable and Lingmell. The steep slopes on the southeastern side of the lake, leading up to the summits of Whin Rigg and Illgill Head, are known as the "Wastwater Screes" or on some maps as "The Screes". These screes formed as a result of ice and weathering erosion on the rocks of the Borrowdale Volcanic Group, that form the fells to the east of the lake, towards Eskdale. They are approximately 2,000 feet, from top to base, the base being about 200 feet below the surface of the lake.

A path runs the length of the lake, through the boulders and scree fall at the base of the craggy fell-side. On the northwestern side are the cliffs of Buckbarrow (a part of Seatallan) and the upturned-boat shape of Yewbarrow. Wast Water is the source of the River Irt which flows into the Irish Sea near Ravenglass.