Diamond Head State Monument
One of Hawaii's most recognizable icons, this volcanic ash cone is impressive pyroclastic stuff—a tectonic memory from an explosion half a million years ago, measuring more than 1,066 meters (3,497 feet) across and with a 231-meter (758-foot) summit. British sailors named it Diamond Head in the 1800s, mistaking calcite crystals in the crater’s soil for jewels. Despite the volcanic grandeur, the ascent isn't tough and takes most hikers just an hour. At the top, on a clear day, you can see forever.