Saiko Bat Cave is the one of the National Monument of Japan and located in Aokigahara Jukai, right next to Lake Sai. Deep side of the caves are preserved for wild bats, but there are stairs, floors and lights are set for visitors. So, you can enjoy small venture in this cave. You have to be careful when you entire this cave because floor is slippy and top is very low.
The admission office will rent you the helmet for your safety.