This observation deck south of the lake offers a fabulous view of the perfectly symmetrical cone of Mt Nantai, an active stratovolcano. During a series of eruptions 20 000 years ago, magmatic flows diverted the river's course at the foot of its south face. The accumulation of water then led to the creation of a natural dam, giving rise to Lake Chuzenji, the highest lake in Japan (1 270m), and the marshy Odashirogahara and Senjogahara Plateaux. Covered with snow in winter, this landscape is dazzling in autumn.