 Kāneʻohe Bay is what remains of the ancient volcano's summit caldera after the slide.The modern Koolau mountain forms Oahu's windward coast and rises behind the leeward coast city of Honolulu — on its leeward slopes and valleys are located most of Honolulu's residential neighborhoods.The volcano is thought to have first erupted on the ocean floor more than 2.5 million years ago.