Swamp gas is a natural phenomenon in which phosphane and methane rising from decaying organic matter ignite, creating globes of blue flame. This often occurs in marshes and swamps.
In 1994, FBI Special Agent Dana Scully suggested to her FBI partner, Fox Mulder, that swamp gas was a possible explanation for lights that Ranheim had allegedly seen after his truck had lost power in Lexington, Tennessee. (TXF: "E.B.E.")
In 1995, while Mulder and Scully were investigating a reported sighting of foxfire on the same night when George Kearns had gone missing, Sheriff Tom Arens told Mulder that foxfire was nothing more than "a ghost story about swamp gas", although the sheriff correctly suspected that the sighting of foxfire in the area had actually been a fire unrelated to swamp gas. (TXF: "Our Town")