Lake Motosu is the third-largest of the Fuji Five Lakes in terms of surface area, and is the deepest, with a maximum water depth of , making it the ninth-deepest lake in Japan. Its surface elevation of is the same as for Lake Shōji and Lake Sai, confirming that these three lakes were originally a single lake, which was divided by an enormous lava flow from Mount Fuji. The remnants of the lava flow are now under the Aokigahara Jukai Forest, and there is evidence to indicate that these three lakes remain connected by underground waterways. The temperature of the water never drops below , making it the only one of the Fuji Five Lakes that does not freeze in winter.