Anime blogger Danny Choo recently checked out the lava caves of Mt. Fuji, including the Fugaku Fuketsu (wind cave), which is cold year round and was previously used as a natural refrigerator, and the Narusawa Ice Cave, which looks to be even colder.
The caves were created by lava, and there are about 80 caves surrounding Mt. Fuji. Only a few are open to the public for unguided tours. More pics in the link below.
Mt Fuji Caves [DannyChoo]