Lake Onuma (Gunma Prefecture)

Located near the summit of Mount Akagi, one of Gunma’s most iconic peaks and long revered as a sacred mountain, Lake Onuma is a mystical caldera lake set at an altitude of 1,350 meters. Surrounded by pristine primeval forest, the lake spans approximately 800,000 square meters and is known for its crystal-clear waters, offering visitors a sense of serenity and calm.

Here, you can enjoy a variety of seasonal activities: kayaking, boating, and camping in summer, smelt fishing in autumn, and ice fishing on the frozen lake in winter. One of the lake’s most magical sights in winter is the appearance of “ice bubbles.” These occur when methane gas from underwater plants freezes into transparent bubbles beneath the ice, creating a surreal, otherworldly scene. While similar phenomena are seen at Lake Akan and Lake Nukabira in Hokkaido, Lake Onuma is a rare and accessible spot in the Kanto region to witness this natural wonder.

For those who prefer a more peaceful experience, the lakeside features a well-maintained 1.5-hour walking trail that offers stunning views year-round—from fresh green foliage in spring to brilliant red and gold leaves in autumn—making it a perfect destination for both adventure and relaxation.