marsh gas 
noun [mass noun]
Methane, especially as generated by decaying matter in marshes.Methane is often called marsh gas because it forms in swamps and marshes from the underwater decomposition of plant and animal material....
- Occasionally a hiss of marsh gas would sound, like a snake's venomous warning.
- The smell of sulfurous marsh gas drifted over the stockade to mix with the smell of disease and spoiling food.