Narusawa Ice Cave: The lava tube brimming with 10-foot-high ice pillars at the base of Mount Fuji

The Narusawa Ice Cave is a lava tube that is brimming with icicles and ice pillars at the base of Mount Fuji in Japan.

Lava tubes are natural tunnels that form beneath solidified lava flows after a volcanic eruption. Lava hardens faster in the top layers of a lava flow, where the molten rock comes into contact with cool air, than in the middle layers, where it stays red hot and fluid. As a result, when a volcano stops erupting, the core of a lava flow drains away while the outside hardens, leaving an empty conduit, or cave.

The Narusawa Ice Cave is one of several caves that formed during a violent eruption of Mount Fuji in A.D. 864. The eruption took place on the northeast side of the volcano, with lava spewing from a new vent called Mount Nagao rather than from Mount Fuji's central summit crater.

The eruption lasted 10 days and created the large lava plain that's now covered by the Aokigahara Forest, a dense woodland also known as the "Sea of Trees." The blast also split a lake in half, carving out two of the five Fuji volcanic lakes.

The ice cave is one of the three largest caves in this area, along with Fugaku Wind Cave and Lake Sai Bat Cave (also called Lake Saiko Bat Cave). Narusawa Ice Cave is 490 feet (150 meters) long and up to 12 feet (3.6 m) high, according to Wind Cave & Ice Cave, the company that manages and offers tours of the cave. The average temperature in the cave is only slightly above 37 degrees Fahrenheit (3 degrees Celsius) due its particular geology, meaning that any moisture is likely to freeze, especially in the winter.

Water that drips from the ceiling of the cave forms stalactites and stalagmites that meet in the middle during the coldest months. The best time to visit Narusawa Ice Cave is in winter or early spring, when these ice pillars can reach up to 1.6 feet (0.5 m) thick and 10 feet (3 m) tall, according to Wind Cave & Ice Cave.

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The cave is a tourist attraction, but historically, it was used to keep seeds and silkworm cocoons cool. People carved rectangular blocks out of the ice pillars, which they then stacked to create an icebox or refrigerator of sorts, according to Wind Cave & Ice Cave.

"To prevent growth of the cocoons and to preserve the quality of seeds and promote budding, they were stored in a refrigerated environment," the company states on its website.

The cave also holds the remains of ancient trees, which were knocked down by lava flows during the A.D. 864 eruption, pictures on the website show.

The Narusawa Ice Cave sits just half a mile (800 m) east of Fugaku Wind Cave, which extends much deeper belowground, boasts impressive lava formations and houses unusual moss colonies, according to the Japan National Tourism Organization. There is no echo in the wind cave because the walls of pure basaltic rock absorb sound — and unlike in the ice cave, there isn't any frozen water for sound waves to bounce off of.

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