The past eruptions of Mount Fuji led to the creation of multiple caves. The most notable is the Bat Cave, Ice Cave, and Wind Cave. These caves can be found near Lake Sai, one of the Fuji Five Lakes. The Bat Cave is almost 350 meters long and was named after the former inhabitants that used the cave for shelter during the winter months. The ice cave is known as the Narusawa Ice Cave and has been used to store ice since the 1900s. It is also designated as a natural monument and has ice pillars that can be seen inside all year round. The Fugaku Wind Cave was used as refrigerator to store silkworm eggs until the Showa era. The cave is designated a natural monument like the Narusawa Ice cave. The basaltic walls in the cave absorb sound, which prevents an audible echo even though it is 201 meters long and 8.7 meters high.