Lake Tazawa (Tazawa-ko) is a caldera lake in Semboku, Akita Prefecture, northern Japan. It is the deepest lake in Japan (the maximum depth is 423.0 m). Because of its depth, it never freezes. Formerly the lake was noted for having very clear water which could be compared to Lake Mashu in Hokkaidō. It was polluted due to the construction of a hydroelectric dam in 1940 and acid drainage from the Tamagawa Hot Spring (onsen). As a result, the indigenous fish kunimasu (Oncorhynchus nerka kawamurae) was thought to have gone extinct, however a living population was found in a different lake, Saiko Lake, in 2010 proving otherwise. The former town of Tazawako was named after the lake. JR East maintains the Tazawako Line. While once the limited express Tazawa ran from Morioka to Akita, it was replaced by the Akita Shinkansen Komachi in 1997.