Tonlé Sap (; Khmer: ទន្លេសាប, Tônlé Sab [tɔnleː saːp]; lit. 'Fresh River' or commonly translated as 'Great Lake'; Vietnamese: Biển Hồ/𤅶壺, Hán văn: 湖海/壺海) is a lake in the northwest of Cambodia. Belonging to the Mekong River system, Tonlé is the largest freshwater lake in Southeast Asia and one of the most diverse and productive ecosystems in the world. It has been designated as a Biosphere Reserve by UNESCO in 1997 due to its high biodiversity. In the 21st century, the lake and its surrounding ecosystems are under increasing pressure from deforestation, infrastructure development and climate change.