From Akita Wiki
Along with Dakigaeri Gorge, Lake Tazawa forms the centerpiece of the Tazawako-Dakigaeri Prefectual Park. At 423.4 meters deep it is the deepest in Japan, and the 17th deepest in the world. The surface of the lake rests at 249 meters above sea level. Because of the its extreme depth, the lake never freezes in the winter. The depth is also responsible for the variety of colors that the lake expresses, from jade green to indigo blue.
Lake Tazawa is "Sister Lakes" with Saiko Lake in Yamanashi, and Chengcing Lake in Taiwan.
The name "Lake Tazawa" was thought ot be given to the lake sometime in the Meiji Era. Before then it was known as Tawaza Lagoon, or Tatsuko Lagoon. The name is said to either come from the meaning of "Lagoon of the Tazawa Village" or from the Ainu, "Hill With a Raised Dome."
Kannon eventually decided to answer the girls fervent prayers and appeared before her. She told Tatsuko to drink of the waters of the lake, and she would be beautiful forever. After she drank from the water, Tatsuko noticed that she became so thirsty that no water could sate her. She realized that she was becoming a dragon, and dejectedly sunk into the lake.
Worried about her daughter, Tatsuko's mother went to the mountains looking for her. After searching for a long time she returned to the lake defeated. At that moment her daughter appeared to her just as she looked before, but her mother knew that it was no longer the daughter that she knew. Tearfully she said goodbye to her daughter, and threw the torch that she was holding into the water. It is said that this torch transformed into the mysterious fish, the kunimasu.
In the north of Lake Tazawa lies Hachirogata Lagoon. In the lagoon there was another man named hachitaro who had also been turned into a dragon. He learned of Tatsuko, and the two eventually fell in love and moved to Lake Tazawa together. After Hachirogata lagoon lost its master it began to become shallower and shallower every year, while Lake Tazawa, now with two masters, stubbornly refused to freeze over even in the harshest of winters.