Spontaneous Combustion

Some materials can ignite without an external spark. Oily rags left in a pile can self-combust: the oil slowly oxidizes and generates heat. If that heat cannot escape, the temperature climbs until it reaches the ignition point.

White phosphorus is so reactive that it bursts into flame the moment it touches air, so it must be stored underwater. Coal mines face a similar danger from methane, which seeps from the seams and forms explosive mixtures with air at concentrations between 5% and 15%. Mine ventilation exists to keep methane below that threshold.