For centuries, miners took canary birds into mines to warn of danger. When a tunnel or shaft collapsed or was blocked and the oxygen supply diminished, the canary reacted first, often dying and alerting miners to trouble so they could leave immediately. Canaries, and sometimes pigeons or mice, were used in coal mines around the world to test for poisonous gases, especially carbon monoxide, which is colorless and has no taste or smell. Because of its extremely rapid heartbeat, a canary would swoon from even very small amounts of carbon monoxide before the gas became fatal to miners, and it often could be revived if evacuated at once with them. Until modern detection devices became mandated, canaries were carried underground in small wood or metal cages, especially after a fire or explosion, to give a clear signal of whether conditions were safe; in the presence of carbon monoxide, the bird would sway on its perch before falling, dramatically revealing dangerous conditions. Over the years, this yellow songbird saved countless human lives.