Kauai is the smallest of the four primary islands, and it's also the oldest. Geologists have determined that a single volcano formed the island around 5 million years ago. There is a reason Kauai is called the Garden Island — it's green nearly everywhere you look. With an average of 444 inches of rain per year (that's 36 feet!), Mt. Wai'ale'ale (5,148 feet) is considered to be the wettest spot on earth. The nearly constant rain on Kauai's mountainous interior creates incredible waterfalls and feeds the Hawaiian Islands only navigable river, the Wailua.