Onami-no-ike is the highest crater lake in Japan. It is located to the southwest of Mt. Karakuni-dake in the Kirishima mountain range. Of 10 lakes in the Kirishima mountain range, this is the second largest lake and one of a few lakes where fish inhabit. The lake fills the deep caldera (1412 m above sea level) that was formed by the eruption of Mt. Kirishima about 40,000 years ago.

During the seasons of tender green and crimson foliage, the area around the lake is crowded with tourists. In winter, beautiful hard rime and migrating bird such as mallards and spot-billed ducks can be seen. Seen from the observatory on the lakeside, the reflected image of Mt. Karakuni-dake in the lake is exquisite itself. As it is a part of Kirishima-Yaku National Park, there are a lot of other sightseeing spots around the lake.