Oppau: BASF used dynamite on 4,500 tonnes of fertilizer — 560 killed, a town destroyed

At 7:32 am on 21 September 1921, a blast tore through the BASF plant in Oppau, Germany, destroying the factory and most of the surrounding town. The explosion of roughly 4,500 tonnes of ammonium sulfate-nitrate fertilizer in a tower silo killed at least 560 people, injured about 2,000 more, and left 6,500 homeless. The blast was heard 200 km away and left a crater 125 metres wide and 20 metres deep.

The fertilizer had hardened into a plaster-like substance after months in storage. BASF workers routinely used small dynamite charges to loosen caked fertilizer — a technique considered safe since a 1919 test had found the mixture stable below 60% nitrate. But BASF had recently changed its manufacturing process, lowering humidity from 3–4% to 2% and reducing density. Modern analysis shows that at 55–60% nitrate the mixture becomes powerfully explosive, and the lower humidity made it even more unstable. A similar incident had killed 19 in Kriewald two months earlier using the same method.

The Oppau explosion remains the worst industrial disaster in German history. It killed more people than the Hiroshima atomic bomb, though that comparison distorts as much as it clarifies — both numbers are around 6 figures before context. The disaster forced a complete re-evaluation of ammonium nitrate safety, and the town of Oppau was rebuilt over years. BASF continued operations at the site.