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The world's longest lave tube formed back when Hallasan was still an active volcano. The first kilometer of the tube/cave has been opened to the public and makes for one of the more interesting sites in Korea. Be sure and dress warm however - no matter the temperature outside the Manjanggul rarely gets above 10 degrees (about 50 degrees F.) on the inside. The path is relatively well-lit but a bit slippery from the humidity.
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Transportation
Car: slightly less than an hour from Cheju/Jeju City- Bus: you'll need to take a Cheju City-Songsan bus and get off at the 'Kimnyong' exit (also marked for the caves) and switch to a local Manjanggul bus. The local buses leave approximately every 30-60 minutes from 7:50 a.m. to 6:30 p.m.