""Lake Mashu"", located in Kawakami-gun Teshikaga-cho of Hokkaido's eastern region. It is in a place that is approx. 11km from Teshikaga-cho and approx. 12km from the Kawayu Onsen. Lake Mashu, which is within the Akan Mashu National Park, is known as the lake which boasts the highest level of transparency within Japan (it is a close second to the Lake Baikal in the world stage).
On top of its high level of transparency, it is also extremely deep (its greatest water depth is 212m), and it does not reflect anything but the color blue, becoming a blue sapphire color called the ""Mashu Blue"".
「""Lake Mashu"" is surrounded by a caldera ring with an average sea level of 600m, making it a crater lake. Its fantastical appearance when covered by hoarfrost is also very famous.
The color of ""Lake Mashu""'s coast seen in the summer is a blue tinged with green. During the winter season the level of transparency goes up in comparison to summer, and its blue contrasting against a world of silver is made more obvious by the senses.

Wintertime Lake Akan, freezing from the period of January’s third half up around the third half of February, as well as having hoarfrost sticking to trees, is beautiful and its scenery expands all around.The appearance of ice freezing in progress is especially beautiful, and the white rime becomes the thing that makes the sapphire blue of the lake surface shine in the spotlight of a silver world. The structure of the freeze varies and depends on the year. Based on the day and if it changes during the morning and the noon, there might be cases where it will freeze from the banks or have a spotted pattern, or maybe even have snow fall or not during the freezing period; not one time is the same as others. To enjoy the ice the of Lake Mashu, we recommend snowshoe hiking from the 1st Observatory going towards the 3rd Observatory. While hiking during days when the temperature cools down to -15 degrees, we might be able to see “Hoarfrost” in the morning. And the pale appearance of said “Hoarfrost” clinging to trees is simply ethereal!“Hoarfrost” might also trickle down piece by piece when past 10:00 AM.As “Lake Mashu” is surrounded by a ring called the Mashu Caldera, it is not uncommon for the temperature to drop down to -20 degrees if the requirements are met, and trees will be cooled and be clung to by “Hoarfrost” if they are in contact with water that then freezes. Though chances of “Hoarfrost” are low should it continue to be sunny, the sun rise seen from the Mashu Ring is also something worthwhile to see and one of its charms. The surroundings of the Observatory becomes touched by “Hoarfrost”, and when they are hit by the rays of the rising sun, you might meet “Hoarfrost” that shine pink like Sakura cherry blossoms.

As moving alone in the outdoors is extremely dangerous, if you want to see the “Hoarfrost” we recommend you the “Lake Mashu Hoarfrost Tours” that includes an accompanying guide. It is held from December until February of the following year. Enjoy the sights of Lake Mashu in its coldest period while listening to the guide’s explanations regarding the surrounding outdoors and “Hoarfrost”. As the temperature during the tour will go down below -10 degrees, please make sure to wear appropriate winter clothing from head to toe before setting out. As the temperature during the tour will go down below -10 degrees, please make sure to wear appropriate winter clothing from head to toe before setting out.Don’t forget disposable hand warmers too. Though it has another name of “Foggy Lake Mashu”, the weather in wintertime Lake Mashu is comparatively stable and there many days where it is sunny. If you are able to see “Hoarfrost”, then we can guarantee that it will be a memory that will last you a lifetime!