This weekend the Mekong River will again erupt with its mysterious fireballs. What are they? Join the never-ending debate and make up your own mind. The phenomenon is as enigmatic as it is beautiful. Thousands of pink and red bursts of light - the "naga fireballs", sometimes called "ghost fireballs" - rise from the Mekong in Nong Khai on the Lao border. It might as well be an episode in "The X-Files" sci-fi television series. It happens only at this time each year, under the full moon of the 11th lunar month, which falls on Monday on the old Laotian calendar.