Lake Tazawa is a caldera lake located to the west of Mt. Komagatake in Akita Prefecture. It is the deepest lake in Japan. Once oligotrophic lake water turned to acidic when a war regime introduced the highly acidic water from Tamagawa Hot Spring in 1940 for power generation and irrigation. Although the Japanese Government constructed a neutralization facility in 1989 full neutralization of the lake is not achieved yet.