Stench gas warning systems give miners the signal to head to shelter during serious emergencies.
Deep underground in a noisy mining environment, it can be very difficult to quickly raise an emergency alarm. It is often very noisy. It may be very dark or there may be many bright lights, or both in turn. There may be no source of electric power. Workers may be isolated from each other, spread over large distances, and separated by thick rock. But there will always be air (mines work very hard to make sure of this) and that air allows mines to use a very interesting way to communicate: Stench gas.
Ethyl Mercaptan has a very distinct, highly unpleasant odor that humans can easily detect and recognize, even when it is present in small amounts. Mine alarm systems (remote or manual) can release the gas, and a non-flammable propellant, into the ventilation system. It spreads rapidly through a large volume of air. When the smell is detected, miners immediately report to designated shelter areas. They remain there until an all-clear is given, which may include a distinct all-clear scent such as wintergreen.
I learned about this while touring the Vale Chasm demonstration mine at the Science North Dynamic Earth attraction in Sudbury, Ontario.