The two volcanoes long thought to have formed the Hawaiian island of Oahu had a head start: They grew on top of an older volcano that’s now submerged northwest of the island and partially covered by it, new research suggests. Rocks forming the northwesternmost point on the island today spilled from Waianae volcano, the older of the two long-extinct volcanoes that make up modern-day Oahu, so ridges extending more than 100 kilometers offshore from there were presumed by some scientists to have originated from that peak as well. But analyses of rocks taken from one of the undersea ridges by teams in the past decade (samples obtained from sites marked with colored dots) show that the ridge’s rocks are up to 1 million years older than Waianae is. Many of those rocks formed above water, which indicates the oldest, now-submerged volcano once stood much taller than it does today. Radioactive dating of the rocks reveals that the oldest lavas on the surface of the ridge, whose peak now lies about 800 meters below the waves, cooled about 5 million years ago, the researchers report online this month in GSA Bulletin. Other tests indicate that the long-lost peak—now dubbed Kaena volcano—grew from the sea floor and broke through the ocean’s surface about 3.5 million years ago, eventually reaching a height of about 1000 meters above sea level before it began sinking back into the sea. At its largest, ancient Oahu would have measured about 1900 square kilometers (about 20% larger than modern-day Oahu) or larger. Over the course of its lifetime, Kaena volcano spilled between 20,000 and 27,000 cubic kilometers of molten rock, the researchers estimate. When Kaena volcano became largely extinct isn’t clear, the team notes. Volcanism likely ceased sometime after 3.5 million years ago, although some of the team’s samples reveal that some minor undersea eruptions occurred as recently as 340,000 years ago.