A glowing, mysterious light that you see at night in a swamp or marsh is an ignis fatuus. An ignis fatuus is caused by a chemical reaction, but it looks ghostly.
The combination of methane gas and decaying plants that's found in bogs and swamps can result in a glow or pale fire that's visible at dusk or in the dark. One of many names for this is ignis fatuus — it's also known as a will o' the wisp or a friar's lantern. There are numerous myths explaining the ignis fatuus, holding ghosts, fairies, and devils responsible for the spooky lights. In modern Latin, ignis fatuus literally means "foolish fire."