There are no rivers flowing into or from the caldera lake, Mashu-ko (摩周湖) is up to 211m deep and once the clearest lake in the world. Due to its rugged shores are prohibited to enter, but there are two viewpoints on the western rim for a view over the lake to the picturesque surrounding areas. The lake often covered in a fog from spring to summer, check the weather condition before you go.
Mashu Viewpoint 1
(摩周第一展望台) Known as the Dai-ichi Tenbodai, the main viewpoint of Mashu-ko is accessible from JR Mashu Station by Akan Bus (2 daily; 25min; ¥570). A parking ticket (¥500: late Nov-early Apr) is also available at Io-zan.
Alternatively, a 6.6km footpath (takes about 2hrs) connects JR Biruwa Station and the Viewpoint 1.
Mashu Viewpoint 3
(摩周第三展望台) This place is 3.4km north from Mashu Viewpoint 1. It is closed in winter. Note: there is no ‘Viewpoint 2’ today.
(摩周岳) Also called Kamuinupuri (カムイヌプリ), a challenging 7.2km scenic trail on the southern rim leads from Mashu-ko Viewpoint 1 (543m) to the top of Mashu-dake (857m). You can overlooking the inky blue lake. The return hike takes around five hours.