Nong Khai celebrates the generic Thai and Isaan festivals with due gusto, but in recent years a peculiarity of this stretch of the Mekong River has been attracting thousands of celebrants from Bangkok and beyond. Every year on the full-moon night in October, silent and vapourless naga fireballs appear from the river, small, pink spheres that float vertically up to heights of as much as 300m, then disappear; in some years, several thousand appear, in others, just a handful. A tentative scientific theory proposes that the balls are a combination of methane and nitrogen from decomposed matter on the bottom of the river, which reach a certain temperature at that time of the year and are released, combusting in the presence of oxygen when they break the water’s surface; romantics will prefer the local belief that the nagas or naks (serpents) of the river breathe out the fireballs to call the Buddha to return to earth at the end of Buddhist Lent.

This strange occurrence has now been consolidated into the two-day festival of Bang Fai Phaya Nak, which coincides with Awk Phansa and the end of the longboat-racing season on the river. The fireballs have appeared as far afield as Sang Khom and Bung Kan, but are generally most numerous at Phon Phisai, 40km east of Nong Khai; if you make the trek out there, take great care on the way back, when the road is thronged with drunk drivers.

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