Swimming back through the Emerald Cave tunnel on Koh Mook

Description stolen from a site:
The Emerald Cave or 'Tham Morakot' is usually the highlight of the famous 4 Island Tour -- a, not to be missed, snorkelling tour or day trip to the Southern Islands, near Trang.
The Emerald Cave is a secret lagoon hidden inside the island of Ko Mook. The only way in to this amazing location is to swim 85m through an underwater tunnel that opens out into the secret bay with limestone cliffs soaring high above you on all sides.
The underwater tunnel turns through about 90 degrees, so you descend into total darkness for a few seconds. Once you complete the turn, you can start to make out the emerald-green light coming from deep within the island. As you exit the tunnel and enter the lagoon, the water glows a deep emerald green and it's rather like being inside a big, green volcano. There's a large sandy beach to relax on, that's open to the sky inside the cave emerald-cave-lagoon
It is a truly amazing experience to emerge from a dark tunnel into a stunningly green tropical paradise, complete with large white-sandy beach. You can't help but imagine what it felt like to discover this, almost film-set like place, for the first time.
The cave used to be a smugglers resting place and is reputed to have been used by pirates as a secret and secluded place to stash their treasure. It retains an air of mystery and romance and it's easy to imagine smugglers hiding out with their loot inside the cave.emerald-cave-sunset
The cave reputedly forms part of Alex Garner's inspiration for The Beach. It's well known that Maya Bay, Koh Phi Phi was used as the beach setting for the film version of The Beach. But, it's much less well known that Ko Mook's Emerald Cave, with it's secret entrance and totally cut-off lagoon, was the inspiration for The Beach's alternative access point.