Marsh gas is a term used to describe the flammable gases that are produced by decaying organic matter in marshes and wetlands. The two crossword solutions for marsh gas are METHANE (7 letters) and poison gas (9 letters). METHANE is a colorless, odorless, and flammable gas that is the main component of natural gas. Poison gas is a type of chemical weapon that is used to cause death or injury through the inhalation of toxic gases.