Mercury
Mercury is a lethal substance if used incorrectly it is deadly. Mercury is most useful if it is a liquid. We use it in thermometers and in power stations.....even in weapon of mass destruction.
There was once a spill on the coast of Japan. Mercury was flowing deliberately into the fresh water river. When Mercury is mixed with water it makes an even more deadly poison Methyl Mercury. The fish in that river were poisoned and died, the fishermen took the fish out of the river, then they sold the poisoned fish and kept some for themselves (because most fishermen were poor). The people ate the poisoned fish and died (so did the fishermen). The factory was sued for 1.5 billion US Dollars for the death of hundreds of innocent people.
Information from the book Chemical Disasters by Rob Alcraft. Des Plaines, Ill. : Heinemann Library, c2000.