Saiko Koumori-ana (Saiko Bat Cave)

Overview

Saiko Koumori-ana (Saiko Bat Cave), located in Fujiyoshida-cho, Yamanashi Prefecture, is a lava cave renowned as a bat habitat. Designated a National Natural Monument, the cave, formed by a Fuji volcano lava flow, showcases unique formations such as lava domes and rope-like lava – characteristic features of lava caves. It offers a thrilling cave exploration experience while also allowing you to enjoy the abundant nature of the Aokigahara Forest. Saiko Koumori-ana was formed by lava flows from the Jogan eruption, which occurred between 864 and 866 AD. With a total length of 386.5 meters, it's one of the largest lava caves in the Fuji Five Lakes area, boasting a complex, three-dimensional structure with multiple branching tunnels. The cave features numerous formations typical of lava caves, including lava domes, rope-like lava, and lava shelves. Lava domes were formed when gases, primarily water vapor, accumulated in one place during the cave's formation. A lava dome approximately 1.5 meters in diameter and 50 centimeters high is found near the center of the cave's largest hall. Rope-like lava, showcasing the direction of the lava flow, is one of Saiko Koumori-ana's most iconic features. This formation occurs when less viscous lava flows on a gentle slope; the surface cools and solidifies while the interior remains molten. The solidified upper layer stretches, creating a ribbed texture similar to multiple ropes twisted together. The cave's stable year-round temperature once supported a large bat population using it for hibernation. However, development around the cave and unregulated access led to a significant decline in the bat population. Since the 1990s, conservation efforts have gradually led to a recovery in the bat population. Two sections of the cave are now protected areas with wooden fences restricting access to safeguard the bats. Saiko Koumori-ana offers more than just cave exploration; it's an opportunity to immerse yourself in the rich natural beauty of the Aokigahara Forest. Well-maintained walking paths wind through the forest, where you can observe the unique vegetation up close, with conifers such as Tsuga, Urajiro-momi, and Goyoma-matsu predominating alongside broadleaf trees including Mizunara, Konara, and Kaede, creating the characteristic dense, dark forest of Aokigahara.