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The Union Carbide plant at Bhopal manufactured a pesticide called Sevin, which works by attacking the nervous systems of insects. The cheapest and, potentially, the cleanest way to produce Sevin involves the use of methyl isocyanate (MIC), a volatile, colourless and very dangerous chemical. MIC is highly reactive, and one of the
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things it reacts with is water.
When water entered the plant's main pipe system (see Deteriorating standards), it was able to flow into the underground tank in which the MIC was stored. It was several hours before workers noticed that anything was wrong.
 When MIC reacts with water, it produces carbon dioxide and heat, in what is known as an 'exothermic reaction'. As the heat of the mixture increases, so the speed of the reaction increases. The reaction of MIC with water is known as a 'runaway': it builds its own momentum and, once it reaches a certain point, becomes uncontrollable.
 Once the reaction reaches a given point, 'trimerisation' occurs. This is a process where three molecules in this case, three molecules of MIC react with each other. At the time of the disaster, the process released yet more heat and led to a rapid increase in pressure within the MIC tank.
As the pressure rose, the concrete MIC tank began to rumble, shake and crack. Just after midnight on 3 December, a pressure-release valve on it blew. MIC gas was released into the atmosphere and a poisonous cloud was carried by the wind over the city of Bhopal.
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