This beautiful limestone tunnel leads 80m into a cave on Ko Muk's west coast. No wonder pirates buried treasure here. You have to swim through the tunnel, part of the way in darkness, before exiting at a small white-sand beach surrounded by lofty limestone walls. A piercing shaft of light illuminates it around midday. National park fees (adult/child 200/100B) apply.
The cave features prominently on most tour itineraries, so it can get ridiculously crowded in high season. It's best to charter a long-tail boat (800B to 1000B) or rent a kayak and zip over at daybreak or late afternoon when you'll have it to yourself, but note that you can’t get inside at high tide.