Located along the Na Pali Coast on the island of Kauai, Hawaii, Waiahuaka Cave is accessible only by zodiac raft or kayak. This remote cave, stretching over 350 meters (1,155 ft.), is the second longest sea cave in the world. It is also known as the Sacred Water Cave and as the Double Door cave – as it has both an entrance and an exit. A fissure in the roof of the volcanic rock allows a waterfall to drop into the amazing cavern. It’s a beautiful sight in the bright sunlight when the sun’s reflection turns the ocean’s water into a myriad of blues and greens.