Koh Mook lies between Koh Ngai and the mainland. Although slightly larger than Koh Ngai, Koh Mook is less developed. This is probably due to the craggy nature of the island. Many of the island's white sand beaches are backed by high cliffs. As a result, there are only a few resorts on the island.
The island's main claim to fame is the so-called "Emerald Cave". The cave is an opening at sea level in one of the cliffs on the west side of the island. It can only be entered by boat at low tide. Once past the narrow opening, the cave opens up to reveal a small cove open to the sky, with white sand beaches, and completely surrounded by limestone cliffs. Tours to the cave are available from most of the islands and resorts around the area. Keep in mind that you can only get in and out of the cove at low tide, so if you stay too long, you'll have to wait for the next low tide to get out.