Diamond Head, located on the eastern edge of Waikiki's coastline, is a famous volcanic crater and the most recognized landmark of Hawaii. Diamond Head is known in Hawaiian as Le’ahi meaning "brow (lea) of the yellowfin tuna (ahi)." It got its English name in the late 1700's when British seamen saw calcite crystals sparkling in the sunshine and thought they had found diamonds.
This broad, saucer-shaped crater was formed about 150,000 years ago during a single, explosive eruption that sent ash and fine particles in the air. As these materials settled, they cemented together into a rock called tuff, creating the crater, and which is visible from the trail in the park. Most of the vegetation and birds were introduced in the late 1800s to early 1900s. The crater is 3,520 feet in diameter with a 760-foot summit.
When the United States annexed Hawaii in 1898, harbor defense became a main responsibility. One of the major defense forts, Fort Ruger, occupied the Diamond Head Crater. An observation deck was constructed at the summit in 1910 to provide target sighting and a four level underground complex was built within the walls of the crater as a command post. A 580-foot tunnel was dug through the crater wall to provide easier access to the Fort. A battery of canons was located within the crater providing complete concealment and protection from invading enemies. The observation deck and underground complex is now abandoned with the advent of radar but evidence of the command post is still present along Diamond Head Trail.
Diamond Head is now a popular tourist spot and hiking destination. A 1.1-km hike leads to the edge of the crater's rim. Signs at the trailhead say that the hike takes 1.5–2 hours round-trip, and recommends that hikers bring water. Although not difficult, the signs also say that the hike is not a casual one: the mostly unpaved trail winds over uneven rock, ascends 74 steps, then through a tunnel and up another steep 99 steps. Next is a small lighted tunnel to a narrow spiral staircase (about 30 steps) inside a coastal artillery observation platform built in 1908. From the summit above the observation platform both Waikiki and the Pacific Ocean can be seen in detail.
The volcano is a symbol of the worldwide recognition of the Hawaiian Islands. Many souvenirs from Hawaii and surf shop logos around the world bear the volcano's distinctive silhouette.