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Hawaii: Waiahuakua, this is the second longest sea cave in the world, stretching 1,155ft. It is known as the Sacred Water Cave and as the Double Door cave – as it has both an entrance and an exit. Kayak tours run through the cave to allow visitors a close-up view of the caverns huge length. A fissure in the roof of the volcanic rock allows water to gush into the amazing cavern.