Kauai (Kaua'i in Hawaiian) is the oldest and fourth largest of the main Hawaiian Islands, in the United States. Known also as the "Garden Isle", Kaua'i lies 105 miles (170 kilometers) across the Kaua'i Channel, northwest of Honolulu on O'ahu. It is of volcanic origin. The highest point is Mount Waialeale, at the center of the island, 5,148 feet (1,570 meters) above sea level. The wettest spot on Earth, with average rainfall of 460 inches (11,648 millimeters) a year, is just east of Mount Waialeale. The high yearly rainfall has eroded deep valleys in the central mountain. It has made canyons with many scenic waterfalls.