The Tonle Sap, also known as Cambodia's Great Lake. While in the dry season the lake extends some 150 kilometres in length and around 20 kilometres in width, new radar satellite imagery has revealed the true extent of the lake's flood. At the peak of the wet season with the Mekong river flowing back up the Siem Reap river, the Tonle Sap can expand to some 250 kilometres long and in places more than 100 kilometres wide, giving a total increase from 2,500 to about 13,000 square kilometres. This natural phenomenon was noted at the end of the 13th century by the Chinese envoy Chou Ta Kuan. Colin Poole, Director of WSC Asia, examines all aspects of this fascinating and beautiful lake - environment, fauna, history, culture and future - accompanied by evocative and memorable photographs by Eleanor Briggs.