Lost Lake also known as Knop/p/s Pond is a reservoir in Groton, Massachusetts, United States. It was formed from three lakes by the headwaters of Salmon Brook. The southern part of the lake is known as Knop's Pond where it is near 30 feet deep. It is stocked with rainbow, brown and brook trout every spring and fall and is home to several species of warm water fish. There is also a boat launch located on the northeastern side of the lake.