Almost half a million people flocked to witness the annual rising of fireballs from the Mekong where the river snakes between Thailand and Laos, but the phenomenon turned out to be a damp squib.

A four-hour downpour from twilight on Monday put the dampers on the show for most of the crowd camped out along a 10-km (six-mile) stretch of riverbank opposite the Laotian capital of Vientiane.

Usually thousands of reddish-pink balls of light are said to shoot up into the sky above the Mekong, which runs through five nations before flowing out to sea in Vietnam.

But this year only a few dozen balls were seen.

The fireballs, called "Bung Fai Phaya Naga," or "Naga fireballs," after a mythical giant serpent, have never been proved scientifically, but some speculate they are the result of natural gases rising from the river bottom.

Others say they are a hoax, while local myth tells of an ancient waterworld beneath the brown waters of the Mekong. Locals say the phenomenon has been occurring for half a century...