Emerald Cave, Koh Mook Koh Mook, also known as the Emerald Cave of the Andaman Sea, is the third-largest island in Trang's waters. In addition to its beautiful beaches, Koh Mook is home to one of Thailand's most unique attractions, the Emerald Cave. This small cave stretches approximately 80 meters, and visitors must float through darkness to reach it, which is only accessible during low tide. Inside, you'll discover stunning emerald-green waters, reminiscent of a hidden gem, leaving visitors in awe of its breathtaking beauty.