Emerald Cave, or Tham Morakot, is the reason many travellers first hear of Ko Muk. The entrance sits in the cliffs on the island's west side, and the inner beach can only be reached when tide and sea conditions allow.
Go early if you can. Midday tours from Ko Lanta, Krabi, and Trang can turn the cave mouth into a queue, while a kayak from Ko Muk gives you a better chance of catching the place in a quieter mood.