The longest and deepest lava tube in the world, over 40 miles of passage running down the flank of Kīlauea on the Big Island of Hawaii, drained by an eruption around 500 years ago. It is a single continuous tube dropping nearly 3,600 feet, with lava falls and stalactites of frozen rock. Parts are shown by private guided tour (it runs under private land); this is real, serious caving, not a lit walkway. Guided access only, near Hilo.
Hawaii
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