Every October, glowing balls rise from the depths of the Mekong River in Thailand and Laos. Each night hundreds of reddish orbs, each the size of an egg, rise to about 200 meters above the river and disappear.

No one’s sure what’s causing them. Possibly the river’s sediment is fermenting and combusting. Villagers attribute the fireballs to the phaya naga, which they say lives in the Mekong. Whatever the cause, they make for a good festival.