Poisonous gas of two sorts.
Choke damp - the carbonic acid gas (or air largely mixed therewith) which accumulates in old workings in coal-pits, and at the bottom of wells, quarries, and caves; after an explosion in a coal-mine, it often rises and mingling with the remaining nitrogen, steam, smoke and dust, constitutes the after-damp. Also known as stythe damp or black damp.
Firedamp -  Flammable gas, esp. methane (marsh gas), given off by coal in mines and which can form an explosive mixture with air. [OED]

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