Then there is .... Bhopal: the Union Carbide gas leak.

http://www.lenntech.com/environmental-disasters.htm

The cause of the accident has been researched after the disaster. Apparently water ended up in MIC storage tanks, causing an exothermal reaction that released an amount of poisonous gas large enough to open the safety valves. Normally scrubbers would intercept escaping gas, but these were temporarily out of order for repair.
Research showed that factory personnel neglected a number of safety procedures. There were no valves to prevent water from entering the storage tanks. The cooling installation of the tanks and the flaring installation that might have flared the gas that was released were out of order (fig. 1).
Safety was very low in this factory of Union Carbide, compared to its other locations. The safety procedures were neglected because of budget cuts......

...In 2001 Dow Chemical Company took control of Union Carbide. These take-overs led to a discussion on responsibility for cleaning up the tons of poisonous waste that are still present in the environment consequential to the 1984 disaster. Environmental activists are trying to convince Dow Chemical Company to clean up this potential minefield of toxic chemicals. These could cause nervous system failure, liver and kidney disease and possibly cancer for many years to come.
Today, the location is still polluted with thousands of tons of toxic chemicals, such as hexachlorobenzene and mercury. These chemicals are stored in open barrels. Rainfall causes rinsing out of pollution to local drinking water sources. According to BBC research, some wells even contain up to 500 times the legal limit of these toxins. Local residents still suffer from a number of diseases, which appear to be very uncommon among people that do not live in the disaster area.

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Nice.