Thale Nai

Thale Nai, also known as Emerald Lake, is a saltwater lagoon on Ko Mae Ko island within Mu Ko Ang Thong National Marine Park in Surat Thani Province. Enclosed by limestone cliffs and connected to the sea through an underground tunnel, the emerald-green lake is the park's most famous natural landmark. It is accessed via a short stair climb to a viewpoint and is visited as part of organized boat tours from Ko Samui.

Visitors arrive by boat to Ko Mae Ko and climb a series of steep stairs to a viewpoint overlooking Thale Nai. The lake appears as an emerald-green enclosed lagoon surrounded by sheer limestone cliffs and dense vegetation, with no direct beach access. The viewpoint has metal railings and platforms for photography. The area is undeveloped with no swimming allowed in the lake itself. The visit is typically combined with island-hopping, kayaking and snorkeling elsewhere in the marine park.

Thale Nai is a natural geological formation within Mu Ko Ang Thong National Marine Park, which was established as a national marine park in 1980. The lake was formed as a collapsed limestone sinkhole that remains connected to the Gulf of Thailand via a subterranean cave system, illustrating karst topography of the region.