 It is not a mountain range in the normal sense, because it was formed as a single mountain called Koolau Volcano ("koolau" means "windward" in Hawaiian, cognate of the toponym "Tokelau").What remains of Koolau is the western half of the original volcano that was destroyed in prehistoric times when the entire eastern half—including much of the summit caldera—slid cataclysmically into the Pacific Ocean.Remains of this ancient volcano lie as massive fragments strewn nearly over the ocean floor to the northeast of Oahu.