Mu Ko Ang Thong National Marine Park was established in 1980 to protect 102 square kilometres of limestone karst archipelago in the Gulf of Thailand. Forty-two uninhabited islands form a chain 31 kilometres northwest of Koh Samui, their sheer cliffs rising 396 metres above emerald lagoons separated from the open sea by natural rock walls. The park's centrepiece, Talay Nai on Ko Mae Ko, is a landlocked saltwater lake fed by underground channels and ringed by vertical limestone that blocks wind and wave action. Surveys in 1997 documented 54 coral species across shallow reefs along the eastern and southern coastlines, though bleaching events in 2010 and 2016 reduced live coverage in the warmest months. The archipelago's name translates to "golden basin," a reference to the bowl-shaped arrangement of islands visible from the summit viewpoint on Ko Wua Talap. Ang Thong Marine Park tour from Koh Samui options include speedboat departures that cover the 19-nautical-mile crossing in 45 minutes and slower cruises requiring 90 minutes under sail. Most itineraries stop at Ko Wua Talap for the 500-step climb to the panoramic overlook, then proceed to Ko Mae Ko for lagoon viewing and Ko Sam Sao or Ko Tai Phlao for snorkelling above brain coral and staghorn formations. The park's protected status prohibits overnight stays, fishing, and reef anchoring, with patrol stations on three islands enforcing these regulations year-round. Tour operators collect the 300-baht conservation fee at embarkation and remit it to the Department of National Parks, which funds reef monitoring and waste removal programmes. Visibility peaks between February and May when southwest monsoon rains subside and plankton blooms thin. The limestone geology dates to the Permian period, when calcite deposits formed under a shallow inland sea 280 million years ago. Tectonic uplift and erosion carved the present karst towers, leaving sea caves, arches, and hongs—collapsed caverns open to the sky—that kayakers now paddle through at high tide. Dusky langurs and white-bellied sea eagles inhabit the forested interior of larger islands, while nesting terns occupy the cliff ledges from April to August. The marine park's isolation has preserved ecosystems that vanished from Samui's developed coastline decades ago, making the archipelago a living reference for what the Gulf of Thailand resembled before mass tourism.