Two hardened tubes are visible in the Ape Cave, a lava tube located near Mount St. Helens in the Gifford Pinchot National Forest in Washington state. The ape cave formed about 2,000 years ago during the only known period when fluid basaltic lava erupted from the volcano. The outside edges of the lava flow cooled first, providing a crust that allowed fluid lava to continue flowing inside. This period of activity may have lasted a year during which the lava level rose and fell, leading to the unique shapes inside the cave. The Ape Cave lava tube is 13,042 feet (3976 meters) long, ranking as the third-longest in North America. The cave is named for a local hiking club, the St. Helens Apes.
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