The Blue Room

The last of three caves on a short hike up from the road and down a steep rocky scramble, Waikapalae wet cave on the island of Kauai is also known as the Blue Room for its secret back grotto, which glowed an otherworldly blue.

The blue of the grotto has been explained as sunlight reflecting off calcite, though some people attribute the blue to a more... out of this world explanation. The cave system in which it is located is associated with a Hawaiian mo’o goddess, a shapeshifting spirit of the deep.

Unfortunately the effect has diminished as the cave's water levels have dropped.  Its unusual blueness is apparently still visible when the tide is highest and the sun is just north of Ha'ena State Park.

Although swimming in Waikapalae is officially discouraged, the unofficial understanding seems to be "do it briefly at your own peril, but not with open wounds, and don't drink the water." (As with other tropical freshwater pools, there's risk of leptospirosis.) Afterward, it's a brief walk to the fully legal swimming at Ke'e Beach. 

The Blue Room also made a cameo as the fountain of youth in 2011’s Pirates of the Caribbean: On Stranger Tides and is also a sort of unofficial sister-cave to Italy's famous Blue Grotto.