Sunset at Tonle Sap Lake, 2018. Tonle Sap is the largest freshwater lake in Southeast Asia. A vast body of water in northwest Cambodia, 3 million people live on or around the lake and make their living from fishing and farming. The lake’s fishing industry is key to the country’s diet and economy. In fact, Tonle Sap Lake is one of the biggest sources of freshwater fish on the planet.