The Oppau explosion occurred on September 21, 1921, when approximately 4,500 tonnes of a mixture of ammonium sulfate and ammonium nitrate fertilizer stored in a tower silo exploded at a BASF plant in Oppau, now part of Ludwigshafen, Germany, killing 500600 people and injuring about 2,000 more.

A silo (from the Greek σιρός – siros, "pit for holding grain") is a structure for storing bulk materials. Silos are used in agriculture to store fermented feed known as silage, not to be confused with a grain bin, which is used to store grains. Silos are commonly used for bulk storage of grain, coal, cement, carbon black, woodchips, food products and sawdust. Three types of silos are in widespread use today: tower silos, bunker silos, and bag silos.