The Koʻolau Range along Oʻahu’s windward side is the eroded western half of Koʻolau Volcano, a single shield volcano that began on the ocean floor more than 2.5 million years ago and became dormant about 1.7 million years ago. In prehistoric time, the volcano’s eastern half collapsed into the Pacific, leaving Kāneʻohe Bay as part of the former caldera and the remaining cliffs and ridges as the range seen today. Later eruptions formed landmarks such as Diamond Head and Punchbowl Crater. The range also shaped Hawaiian history, including Kamehameha the Great’s 1795 victory at Nuʻuanu, and remains a defining natural barrier and landmark of Oʻahu.