A lava tube or tunnel is created when super-hot lava flows from a volcanic eruption. The inner core remains extremely hot, but the surface of the lava cools rapidly when it contacts either air or water. The cooling lava creates a quickly hardening “roof” while the hot inner core flows onward. When the lava disburses at the far end, it leaves behind a tunnel.
Tomie has many lava tubes, but Iana is the largest one, stretching over 1,400 meters in total. Approximately 400 meters from the entrance, it narrows to about one meter and is filled with water.
Within this structure, researchers have discovered a variety of cave creatures that are either rare or unique to this locale, such as the fish, Lucigobius albus, a small, blind species of goby, which is in danger of extinction. It has been discovered in only one other place in all of Japan.
The Iana Lava Tunnel is not open to the public, but the large entrance is a popular tourist attraction.