Despite being known as the Bat Cave, this 350m-long complex has neither bats nor caped crusaders. Nevertheless, it's still fun to scramble over the ancient lava flows, and in the car park you'll find a trailhead for marked hikes through Aokigahara Forest, including a 3km tramp to Fugaku Fuketsu. The green-line bus stops here.
A free museum by the entrance is dedicated to a fish, the kunimasu, a deepwater trout discovered in Sai-ko in 2010, 70 years after it was believed to be extinct.