swamp gas

• (hydrogen sulfide): So called because it is often emitted from wetlands such as swamps as a result of anaerobic bacteria, and is responsible for their foul “rotten egg” odor.
• (ufology): An explanation given by then-skeptic J. Allen Hynek for an alleged UFO sighting in Michigan near Lake Huron in 1966.

swamp gas (uncountable)

1. (literally) Gases emitted from a swamp.

   Synonyms: bog gas, marsh gas, biogas

2. (inorganic chemistry, colloquial) Hydrogen sulfide.

   Synonym: rotten egg gas

3. (ufology, sarcastic) A skeptical explanation that is provided in order to debunk a UFO sighting that a believer regards as ludicrous, illogical or disingenuous.

   Synonyms: weather balloon, pelican