Look at the map and the shape is startling. Thailand's territory here has been squeezed to a thin coastal shelf: the Gulf of Thailand on one side, and on the other, roughly two kilometres inland, the Cambodian border running parallel along the base of the Cardamom Mountains. The whole district covers just 50.2 square kilometres, packed at nearly 478 people to each of them. The hills immediately behind the town reach about 300 metres; a little further north the same range climbs past 1,000. One road serves the entire strip. Route 318 runs south to the border crossing at Hat Lek and north to Trat, where it joins Route 3 - the Sukhumvit Road - for the long run to Bangkok. There is no alternative route, no bypass, and nowhere much to spread out.

The mountains that pin the town against the sea also soak it. Khlong Yai has a tropical monsoon climate, and the Cardamom range standing directly behind it forces the southwest airflow upward and wrings it out on the way over: the town records about 4,845.8 millimetres of rain in an average year, spread across roughly 196 rainy days. No month is genuinely dry. December and January come closest, the rain builds through February to April, and from May to October the real monsoon settles in - more than 800 millimetres in each of June and July, and over 1,000 in August alone. In the 1961-1990 record, one August delivered 2,072.4 millimetres by itself. The temperature barely moves by comparison: 27.3°C on average, a March record high of 38.2°C, and a December morning in the archive that dropped to 15.8°C. That relentless water is why dense rainforest grows on a coastal shelf this narrow.

This coast has not always been Thai, and not for very long without interruption. Khlong Yai belonged to the old province of Patchan Khiri Khet, and in 1904 most of that province - Trat included - was handed to France in exchange for French troops withdrawing from Chanthaburi. Three years later the arrangement was unwound in part: France returned Trat and Khlong Yai to Siam, taking Siem Reap and Battambang in trade, and keeping Koh Kong just across the new line. The administrative machinery followed slowly. Khlong Yai became a minor district in 1912 and was not raised to a full district until 1959. Three subdistricts and twenty villages sit on the strip today, with townships at Khlong Yai itself and at Hat Lek on the border.

When Vietnamese forces pushed the Khmer Rouge out of Phnom Penh at the start of 1979, the war did not end; it moved west into the Cardamoms and along this border, and civilians moved with it. Many of the people who reached Khlong Yai had already lived through the years of the Khmer Rouge regime and then walked out through the same mountains you can see from the main road, carrying whatever they still had. The Thai Red Cross ran the camp that received them from 1979 until July 1986, and at its peak more than 90,000 people were living in it - nearly four times the district's own population, in a place with fifty square kilometres to its name and one road in or out. Some were resettled abroad. Many eventually went home to a country still at war with itself. The centre that stands on the site now carries a different name and a quieter purpose, and most people driving Route 318 to the border pass it without knowing what it held.