Fern Grotto Introduction: Fern Grotto is located in the eastern part of Kauai. It is one of the most popular attractions on Kauai and is also a popular location for wedding ceremonies. The cave was formed by natural lava and is filled with hanging ferns and tropical plants. It was once the royal garden of Hawaiian royalty. Because it can only be reached via the Wailua River, boats have been ferrying tourists back and forth for more than 50 years.