The legendary emerald cave is on Koh Mook . At low tide, a cave opens up from the sea. You will swim around 100 meters through this cave (pitch black inside, but your guide will have a light) and end up at a small, hidden cove, with a white sand beach and surrounded by towering, rain forest covered cliffs, the cave being the only access. Pirates used to hide their booty here, for obvious reasons. When looking from the cave towards the cove, the water takes on an almost fluorescent emerald glow; hence the name.