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The Oppau explosion occurred on September 21, 1921 when a tower silo storing 4500 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 2000 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-Bosch 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. 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.

Crater after the Oppau explosion

The explosion was estimated be equivalent to about 1–2 kilotonnes of TNT and was heard as a loud bang in Munich, more than 300 km away. The pressure wave ripped roofs off up to 25 km away and destroyed windows even farther away. In Heidelberg (30 km from Oppau), traffic was stopped by the mass of broken glass on the streets.

About 80 percent of all buildings in Oppau were destroyed, leaving 6500 homeless. At ground zero a 90 m by 125 m crater, 19 m deep, was created.

According to some descriptions, only 450 tonnes exploded, out of 4500 tonnes of fertilizer stored in the warehouse.

• Deutsche Welle (in German)