Lake Mashu is a caldera lake in Akan National Park.  It is considered the most beautiful lake in Japan and renown for its clear blue water. It’s name “mashu” means mysterious because it is frequently blanketed in heavy fog. Visitors are not allowed to go down to the lake. However, there are two observation decks positioned around the rim of the caldera where you can view the lake. There are facilities at the observation deck where you can take a rest, get some food and drink and go to the restroom.