Le Mans (1955)

As the remains of the 300 SLR slowed its somersault the fuel tank, situated behind Levegh's seat, ruptured. The ensuing fuel fire raised the temperature of the remaining Elektron bodywork past its flashpoint, which due to its high magnesium content was already very low. Magnesium's properties mean that a combustion in oxgyen is possible at relatively low temperatures, allowing the alloy to burst into white hot flames, sending searing embers onto the track and into the crowd. Rescue workers attempting to put out the burning wreckage were initially unsuccessful, as they unknowingly used water on the magnesium fire, which only intensified the inferno. As a result, the car burned for several hours. In total, 82 spectators were killed either by flying parts or from the fire. (Wikipedia: 1952 Le Mans Disaster)