Lake Sai Bat Cave

Lake Sai Bat Cave (, "Saiko Kōmori Ana") is the largest of the several lava tubes that are near Lake Sai, in the Aokigahara forest in the northern side of Mount Fuji, Japan. It is known as a cave where visitors can observe the bats which live there.
Caves in Aokigahara Forest.
In the Aokigahara forest, that was created by the results of the lava flow of the 864 A.D. eruption of Mt. Fuji, there are several lava tubes, of which the major three are: 
All three were designated as Natural Monuments of Japan in 1929. 
Lake Sai Bat Cave.
The Lake Sai Bat Cave is 386.5 meters long and is the largest of the three major caves in Aokigahara. It is located at 925 meters above sea level. 
Bats.
The visitors to the Saiko Bat Cave can observe about 500 bats which live in there. The bats are of five kinds::
Lake Sai Nature Center.
Lake Sai Nature Center was built recently next to the entrance of the Bat Cave. Walking tours through Aokigahara Forest can be arranged with the Fujikawaguchiko, Yamanashi-approved tour guides.
Next to the Nature Center is the exhibition hall of "Kunimasu" trout which was declared extinct ca. 1945 up north in Lake Tazawa, Akita Prefecture, but was discovered in Lake Sai in 2010.
Transportation.
Public bus transportation is available from Kawaguchiko Station of Fujikyūkō railway. By car, 13.1 km (about 25 minutes) on Japan National Route 139, from Lake Kawaguchi exit of Chūō Expressway.