The Oppau explosion occurred on September 21, 1921 when a tower silo storing 4,500 tonnes of a mixture of ammonium sulfate and ammonium nitrate fertilizer exploded at a BASF plant in Oppau, now part of Ludwigshafen, Germany, killing 500–600 people and injuring about 2,000 more.
The plant began producing ammonium sulfate in 1911, but during World War I when Germany was unable to obtain the necessary sulfur, it began to produce ammonium nitrate as well. Ammonia could be produced without overseas resources, using the Haber process.
Compared to ammonium sulfate, ammonium nitrate is strongly hygroscopic, so the mixture of ammonium sulfate and nitrate clogged together under the pressure of its own weight, turning it into a plaster-like substance in the 20 m high silo. The workers needed to use pickaxes to get it out, a problematic situation because they could not enter the silo and risk being buried in collapsing fertilizer.
To ease their work, small charges of dynamite were used to loosen the mixture. The procedure was tried experimentally and was considered safe; it was not known at the time that ammonium nitrate was explosive. Nothing extraordinary happened during an estimated 20,000 firings, until the fateful explosion on September 21. As all involved died in the explosion, the causes are not clear. A theory is that the mixture changed and a higher concentration of ammonium nitrate was present.
It is a Wednesday, 7.32 clock. In Munich, two dull thuds can be heard. Not very loud, but loud enough to worry about what may well be behind it. The answer is not known until hours later:
Some 300 miles away, in Ludwigshafen on the Rhine, has an immense explosion occurred: the disaster site is the site of the Baden Aniline and Soda-Fabrik (BASF) in Oppau, a northern-site of Ludwigshafen. At one point the explosion crater formed by 90 meters wide, 125 meters long and 20 meters deep. 561 people are killed and 1952 injured.