Diamond Head State Monument

Diamond Head, known in Hawaiian as Lēʻahi, is a 232-meter-high (761-foot) tuff formation and the most distinctive landmark of Honolulu and Waikīkī. Its English name dates back to 19th-century sailors who mistook calcite crystals for diamonds. Geologically, the mountain formed about 300,000 years ago in an eruption in which magma was broken into ash by water and steam and cemented into a broad, saucer-shaped crater. The crater covers an area of roughly 1.5 square kilometers (0.6 square miles). The southwest rim is higher because the wind blew the ash in that direction as the crater formed. Since then, rain, wind, and the sea have shaped the slopes, which are now partly protected by a coral reef. In 1986, the site was designated a National Natural Landmark as an outstanding example of a tuff ring.

Today, visitors experience Diamond Head above all as a popular hiking destination. A trail leads up stairs and through the Kāhala Tunnel, dug in the 1940s, to the fire control station at the summit. From there, you get a sweeping panorama of the coast, from Koko Head to the Waiʻanae Range. The military history is still easy to see in the landscape, as the mountain was used for coastal defense starting in 1904. Remnants include underground batteries, the Kapahulu Tunnel, and the four-story fire control station, which was camouflaged with rubble stone and has slits facing out to sea. These installations are part of the Fort Ruger Historic District.

The vegetation in the semi-arid crater is sparse and today consists mostly of introduced species such as the kiawe tree, which came to Hawaii in the 19th century as livestock fodder. Native Hawaiian plant species have become rare. The crater floor once held a small lake inhabited by native birds. Today, however, cardinals, doves, and sparrows dominate. In Hawaiian mythology, the name Lēʻahi is linked to the summit's resemblance to the forehead of an ahi fish, or to the "fire headland," since navigation fires were once lit at the summit.