Lake Mashu
Lake Mashu is in a caldera (large volcanic crater) a located in the eastern section of Akan-Mashu National Park. In the Ainu language, Lake Mashu is called Kamui-to which means “the lake of the gods.” Lake Mashu has an unchanging water level, due to the fact that no rivers flow into or out of the lake. The lake is also known for its natural beauty—especially the unique color of the water dubbed “Mashu Blue.” With a small presence of plankton, Lake Mashu has been able to maintain its pristine waters: since the year 2000, the lake has had an average transparency level of about 20 meters.
Observation Deck No. 1
Lake Mashu Observation Deck No. 1 is on the southwestern edge of the Mashu Caldera. From this observatory, visitors can take in the sheer size of the caldera. Formed by volcanic eruptions approximately 7,000 years ago, Lake Mashu is surrounded by steep walls between 200 and 300 m high.
This observatory marks the trailhead to two nearby peaks. Mt. Mashu (Kamui-nupuri) is the partially collapsed explosion crater visible directly across the lake. It takes about three hours to reach the 857 m summit of Mt. Mashu. Following the same trail part of the way, it takes about two hours and 45 minutes to reach the 799 m summit of Mt. Nishibetsudake. On clear days, both mountaintops offer a 360-degree view of the Akan and Shiretoko mountains, as well as the Mashu Caldera.
Observation Deck No. 3
Lake Mashu Observation Deck No. 3 is located along the western edge of Mashu Caldera and has separate west- and east-facing observatories. Mt. Io is visible from the west observatory, with Lake Kussharo beyond. The east observatory offers a closer view of Kamuishu Island, the small island in the center of Lake Mashu. This viewpoint is also the best place to see the natural phenomenon known as unkai (sea of clouds) that sweeps over the caldera early in the morning, typically between June and October.
The distance between Observation Deck No.1 and No. 3 is 3.4 km. There is a walking trail between the two observation decks as well as a road. Please be careful of the traffic if you walk on the road. Due to snow cover, the road is closed from November until the end of April.
While Observation Deck No. 1 has a rest area with bathrooms and food services, Observation Deck No. 3 has no facilities.