Hokkaido Official Tourism Site HOKKAIDO LOVE!

Lake Mashu is a mysterious lake surrounded by the pristine nature of Akan-Mashu National Park. The deep blue glow of the lake attracts many visitors. The air is also clean, making it a great place to watch the stars.

Lake Mashu is a caldera lake with a circumference of approximately 20 km and maximum depth of 212 m, and which has one of the highest degree of water clarity in the world. The color of the sky is reflected on the clear lake surface, and the deep, beautiful blue reflection is called "Mashu Blue." While visitors cannot go near to the lake surface, they can use three observation decks to view the blue lake, Kamuishu Island floating in the lake, and Kamuinupuri (Mt. Mashu) towering over the sides of the lake. The lake is shrouded in fog for about 100 days every year, but Lake Mashu remains an incredible sight even in the fog! 

Head to the Mashu No. 1 Observation Deck and prepare yourself for the stunning body of water that lies beneath. The uniqueness of Lake Mashu, considered to be one of the most beautiful lakes in Japan, fittingly comes with its own namesake color: Mashu Blue. While the color does change slightly depending on the time of year, such as the sublime sapphire-tinted water often seen in Winter, you will be amazed all year round. For a different kind of landscape, you can enjoy the lake covered in mist and witness the otherworldly “sea of clouds” phenomenon in the early morning from June to October.