Because marshes tend to release methane.
Methane is called a marsh gas because it is formed by methanogenic organisms that can be found is marshes (thus concentrations can be found in marches).
Marsh gas is another term for methane, a colorless and odorless gas that is produced by the breakdown of organic matter in oxygen-deprived environments such as swamps, marshes, and landfills. Methane is a potent greenhouse gas and is also commonly used as a fuel.
Methane
The chemical formula for marsh gas, also known as methane, is CH4. It is a simple hydrocarbon compound composed of one carbon atom bonded to four hydrogen atoms. Methane is a colorless and odorless gas that is the main component of natural gas.
Marshall gas I've not heard of, but Marsh gas is methane CH4
Marsh gas, commonly known as methane, has the chemical formula CH₄. It is the simplest alkane and consists of one carbon atom bonded to four hydrogen atoms. Methane is a major component of natural gas and is produced through the decomposition of organic matter in anaerobic conditions.
The scientific name for methane gas is just methane. The molecular fomula is CH4 . Methane is also known as natural gas, methyl hydride, marsh gas, biogas, fire damp, and R 50 (refrigerant).
Marsh gas is mostly methane. While it is not toxic, it can result in asphyxia if too much oxygen is displaced from the atmosphere.
Methane was discovered in 1776 by Italian physicist Alessandro Volta when he collected the gas produced by decaying organic matter in a marsh. He then named the gas "marsh gas" or "flammable air" before it was later identified as methane.
The chemical name for marsh gas or sewer gas is methane (CH4). It is a colorless, odorless gas that is highly flammable and is produced by the decomposition of organic matter in anaerobic conditions such as marshes and sewers.
Dalton called the gas he collected from ponds "marsh gas" or "carbonic acid gas." Today, we know this gas as methane.
If you see gas bubbling up out of a swamp or marsh, you can suspect that methane may be present in the water. Methane is a common gas produced by the decomposition of organic matter in wetland environments.