Physical and geographical characterization of Lake Drūkšiai is presented. Before the INPP launch, Lake Drūkšiai was a mesotrophic water body with oligotrophic traits. From 1984 to 2010, the lake was utilized as a cooler of the INPP. This chapter reveals how the overall trophic status of the lake, its thermal and hydrochemical state changed, and its pollution increased during various stages of the INPP operation.