Three hundred metres. That is the gap between Koh Thmei's western shore and mainland Cambodia at the narrowest point — a distance a decent swimmer covers in a few minutes, close enough that people on either side can watch each other work. Nobody crosses it. Every boat to the island comes the long way instead, three kilometres down the Prek Toek Sap estuary from the fishing village of Koh K'chhang to the north. The island's name means New Island. It is one of the least new-looking places in the Gulf of Thailand.
Seen from above, Koh Thmei is roughly pentagonal: four sides of similar length and a shorter, rugged fifth facing south. It runs 8.8 kilometres end to end and between five and seven and a half across, with 28.5 kilometres of coastline, and its two shores could hardly be less alike. The landward side is mangrove marsh, an unbroken tangle of stilt roots and tidal channels. The seaward side has real beaches, coarse-grained and thick with shells. In the centre two peaks rise above a hundred metres, separated by a river called the Prek Koh Krabei, and while that elevation is modest it turns out to be decisive — high enough to catch and hold rainfall, so the island runs its own creeks and small rivers rather than drying out between monsoons. That single hydrological fact explains the vegetation, and the vegetation explains everything else about the place.
Every account of Koh Thmei begins with the birds. Some guides put the count at 155 species and others at 'over 150', and the discrepancy matters less than what any such number implies: mangrove, freshwater creek, open beach and primary forest all stacked within a few kilometres. The brahminy kite is the one people come for, chestnut-bodied and white-headed, working the tideline. Beneath the canopy there are monkeys, civets and lizards, and in the wet margins where stream meets mangrove, the fishing cat — a stocky, short-tailed feline that swims, dives and takes fish with its paws, and that depends entirely on exactly this kind of edge habitat. Koh Thmei and neighbouring Koh Seh together form the eastern third of Ream National Park, created in 1993 across 210 square kilometres of forest, mangrove and sea.
Under two hundred people live here. Chong Thmei in the northeast is the largest settlement; Taly sits on the southeastern coast; a small farming hamlet, a bungalow resort and an eco-lodge occupy the west. There is a weather station at Chong Thmei, which is the closest thing on the island to public infrastructure. Roads do not exist, sealed or otherwise, and neither do motor vehicles. The transport hubs are wooden piers, and the timetable is whatever the tide and the boatman agree between them. Visitors who make the trip describe Koh Thmei in superlatives that have nothing to do with comfort — the density of the forest, the quiet, the sense of a coastline nobody has rearranged. Among Cambodia's islands it has a reputation as the one that got left alone.
That reputation is under negotiation. Development companies began taking long land-lease concessions inside the park in 2010, announcing large-scale plans that included a bridge to the mainland; by August 2014 not a thing had been built. In 2011 lawmakers issued an update on Ream and other protected areas designating 84.5 hectares in Ream commune, Prey Nub district, as community land, with any title requiring prior approval from the Ministry of Environment. Then the pace changed. In 2016 Yeejia Tourism Development, a subsidiary of the Chinese-owned Unite International (Cambodia) Investment Group, signed memoranda with more than a dozen mostly Chinese firms for its Golden Silver Gulf resort project, while the gaming junket operator Jimei International Entertainment told the Hong Kong Stock Exchange it had a framework agreement to build a complex of hotels, golf course and theme park. Sub-decrees dated 2 November 2018 then earmarked more than two thousand hectares of Koh Thmei as development zones, to be built out by HTTH Island Investment and Celestial Stars Limited, a casino and lottery company. The national park boundary never moved. The land inside it changed hands regardless.
Koh Thmei lies at 10.481°N, 103.772°E in the Gulf of Thailand, immediately southeast of the Sihanoukville headland and forming, with Koh Seh, the eastern third of Ream National Park. From the air it is a clear pentagon roughly 8.8 km across, dark with unbroken forest, its two low summits just over 100 m and its landward coast blurred into mangrove where the Prek Toek Sap estuary meets the strait — at the narrowest point only 300 m separates island from mainland. Pale shell beaches mark the seaward side. There are no roads, runways or lit structures on the island, so it goes fully dark at night and makes an excellent unlit coastal waypoint. Sihanouk International Airport (VDSV / KOS) is about 25 km northwest with a 3,300 m runway; Phu Quoc International (VVPQ / PQC) is roughly 30 km southeast across the Vietnamese boundary. The Ream naval facility sits on the mainland to the northwest — check current airspace restrictions. Fly 1,500–3,000 ft for detail on the mangrove channels; November to March gives the clearest air, while the July–September monsoon brings low ceilings and squalls.