The colorful Naga fireballs arrive in Laos every autumn, during the full moon at the end of Buddhist Lent. Local scientists think they know what causes the lights, but they don’t understand why they return at the same time every year.

Supalak Ganjanakhundee and Pennapa Hongthong write in The Nation that hundreds of thousands of people come to the area each year to see the orbs, which are about the size of tennis balls. They shoot up into the sky from the surface of the Mekong River, then vanish. One theory is that they’re a hoax, caused by soldiers firing tracer bullets from the other side of the Mekong River. But scientists think they’re a natural phenomenon, caused by a biochemical process. Laos government official Bountiem Phissamay says, “What we can say is it is not a man-made phenomenon. We need to study how it happens.”

A team of 20 chemists collected water, soil and gas samples from the area that produces the fireballs for testing. Several unique compounds were found in the area, including phosphine gas, which is produced from the phosphorus in the earth and water. There is also a high concentration of methane gas in the area, and the combination of the two could cause fireballs. Disintegrating biological matter may also play a role. But it’s still unclear why they shoot into the air in so many colors?and why it only happens for a short period of time every fall.

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