Mae Koh Island is one of the most geographically significant islands within the Mu Ko Ang Thong National Marine Park, renowned for the spectacular Thale Nai or the Emerald Lagoon. This breathtaking natural phenomenon is a large saltwater lake nestled in the heart of the island, completely encircled by sheer limestone cliffs. The lagoon is approximately 200 meters wide, 250 meters long, and has an average depth of 7 meters. Its striking emerald-green water is remarkably clear, reflecting the surrounding lush vegetation. Although it appears isolated from the ocean, Thale Nai is connected to the outer sea via a network of subterranean tunnels and crevices, allowing the water levels to fluctuate in harmony with the tides of the Gulf of Thailand.

Geologically, the lagoon was formed through the collapse of an ancient limestone cavern, known as a sinkhole. Over millions of years, the cave ceiling gave way, creating a massive bowl that was eventually reclaimed by the sea. The ecology around Koh Mae Ko is equally fascinating, consisting of dry evergreen forests that serve as a habitat for diverse wildlife, including Dusky Langurs, hornbills, and various reptiles. To facilitate tourism while preserving the environment, the National Park has constructed a well-engineered staircase system made of steel and wood. These stairs wind up the limestone ridges, leading visitors to a "premier viewpoint that offers a panoramic" vista of the lagoon on one side and the stunning cluster of 42 islands scattered across the turquoise sea on the other.

As visitors descend the path toward the water’s edge inside the lagoon, a wooden boardwalk allows for a close-up encounter with the serene environment. The silence within the crater, broken only by the sounds of nature, creates a powerful sense of tranquility. To protect this delicate ecosystem and its unique water chemistry, the National Park strictly prohibits swimming or any water sports inside the lagoon. Koh Mae Ko stands as a testament to the Earth’s transformative powers, offering a rare opportunity to witness a "living museum" of geological history. It remains a highlight for travelers seeking both adventure and a deep connection with Thailand’s pristine natural heritage.

• Tourists can purchase one-day trip tours from both Koh Samui and Koh Phangan. Most are by speedboat or larger tour boat, with a travel time of approximately 1-1.5 hours. The boat will dock in front of Koh Mae Ko, from where visitors will need to walk up the stairs provided by the national park authorities.

• Wear sturdy sneakers or closed-toe shoes with good grip as the stairs are steep and narrow.

• Carry a small bottle of water to stay hydrated during the climb to the panoramic viewpoint.

• Bring a handheld fan or a folding fan as the air can be quite humid and still on the trail.

• Dress in lightweight, breathable clothing that allows for a full range of motion for climbing.

• Apply sunscreen and bring a hat or umbrella as the top observation deck has no shade.

• For the best photos of the emerald water, visit around midday when the sun shines directly down.

• Strictly follow the "Leave No Trace" policy and carry all trash back to the boat or mainland.

• Secure your camera and phone in a bag while climbing to prevent them from dropping off cliffs.

• Avoid making loud noises to prevent disturbing local wildlife like Dusky Langurs and birds.

• Check weather and sea conditions beforehand as the park may close during heavy monsoon swells.

Park Entrance Fee:

• Thai citizens: Adults 40 baht / Children 20 baht

• Foreigners: Adults 300 baht / Children 150 baht

Opening Hours:

• Open daily: 08:00 – 17:00 (Last admission according to tour boat schedules)

• Note: Closed during the monsoon season, November – December of each year