Some fires along coal seams are natural occurrences. Some coals may self-ignite at temperatures as low as 40 °C (104 °F) for brown coal in the right conditions of moisture and grain size. The fire usually begins a few decimeters inside the coal at a depth in which the permeability of the coal allows the inflow of air but in which the ventilation does not remove the heat which is generated. Self-ignition was a recognised problem in steamship times and is claimed to have contributed to the sinking of the Titanic. One well known source of fires is mining breaking into a high pressure cavity of methane gas which on release can generate a spark of static electricity to ignite the gas and start a coal explosion and fire. The same gas static is well known in ships and care has to be taken to ensure no such static sparking can occur.