Kauaʻi (in stanard Hawaiian pronounced [kauˈaʔi]; in Kauaʻi-Niʻihau dialect, [tauˈaʔi]; usually spelled Kauai outside the Hawaiian Islands and pronounced /ˈkaʊɑɪ/ or /kaʊˈɑɪ/) is the oldest of the main Hawaiian Islands. With an area of 552.3 square miles (1,430.5 km²), it is the fourth largest of the main islands in the Hawaiian archipelago and the 21st largest island in the United States. Known also as the "Garden Isle", Kauaʻi lies 105 miles (170 kilometers) across the Kauaʻi Channel, northwest of Oʻahu. Of volcanic origin, the highest peak on this mountainous island is Kawaikini at 5,243 feet (1,598 m). The second highest peak is Mount Waiʻaleʻale near the center of the island, 5,148 feet (1,569 m) above sea level. One of the wettest spots on Earth, with an annual average rainfall of 460 inches (11,700 mm), is located on the east side of Mount Waiʻaleʻale. The high annual rainfall has eroded deep valleys in the central mountains, carving out canyons with many scenic waterfalls.