Diamond Head State Monument protects and preserves the tuff cone formed by subterranean explosions thousands of years ago. This site has been designated significant by many cultures on the island. The Hawaiians used the summit for human sacrifices. The United States used it as a strategic arms position. Today the site offers recreational attractions. The Hawai'ian name for Diamond Head is Le'ahi. In Hawai'ian legend, it is said that Hi'iaka, sister of the fire goddess Pele, gave Le'ahi its name because the summit resembles the forehead (lea) of the 'ahi fish. Another translation is fire headland and refers to the navigational fires that were lit at the summit to assist canoes travelling along the shoreline. Today, the Diamond Head Light, built in 1917, provides a visual aid for navigation. In the late 1700's, Western explorers and traders visited Le'ahi and mistook the calcite crystals in the rocks on the slope of the crater for diamonds. Thus the name Diamond Head came into common useage. The pronounced seaward summit, deeply eroded ridges, and ovoid-shaped crater are evidence of Le'ahi's very dynamic geological history. The creation of O'ahu began around 2.5 to 3 million years ago with volcanic eruptions from 2 shield volcanoes. Le'ahi is believed to have been created about 500,000 years ago during a single, brief eruption. The broad, saucer-shaped crater covers 350 acres with its width being greater than its height. The southwestern rim is highest because winds were blowing ash in this direction during the eruption. Today, Le'ahi is the most recognized landmark in Hawai'i. It was designated a National Natural Landmark in 1968 as an excellent example of a tuff cone.