Grafham Water is an 806.3-hectare (1,992-acre) biological Site of Special Scientific Interest (SSSI) north of Perry, Huntingdonshire. It was designated an SSSI in 1986. It is a reservoir with a circumference of about 16 km (10 mi), is 21 m (69 ft) deep at maximum, and is the eighth largest reservoir in England by volume and the joint third largest by area at 6.27 km2. An area of 114 ha at the western end is a nature reserve managed by the Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire.