For centuries, people have reported seeing moving balls of light, usually in swampy areas; these have been called swamp lights, will-o-the-wisp, or jack-o-lanterns. Debunkers claim that these are nothing more than glowing or burning methane gas released by rotting vegetation in swamps. While there is no doubt that methane is produced this way, there is no known chemical process that would cause methane to coldly luminesce. And for it to burn, there would have to be something to ignite the gas, and most swamps don't come equipped with spark plugs. If bubbles of methane were somehow ignited, they would simply produce a brief flame just above the water level. Yet scores of witnesses have reported discrete balls of light moving through the air, and holding themselves together for many seconds. Burning gas expands, rises, and cools. Ever turn on the gas range in your kitchen and have to chase a fireball around the room? I think not. In addition, many observers have reported a seeming awareness on the part of the lights; they seem to react to a human presence.
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Reference: ufo-chronicles.blogspot.com