Sasthamcotta Lake, situated around 19 km away from the Kollam town, is the largest freshwater lake in Kerala. This lake is believed to have been named after the old Sastha temple located near the lake. It is regarded as a major drinking water source for people living in the Kollam town. There is a large population of larva cavaborus inhabiting this lake which consumes bacteria from the lake water, making it pure for drinking.