1. 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 561 people and injuring about 2,000 more. About 80 percent of all buildings in Oppau were destroyed.

  2. A Prussian act to protect trees is passed in 1922, with positive effects in conurbations, above all.

  3. Accident insurance is extended, at first to cover 11 occupational illnesses.

  4. The first German Nature Conservation Day (Deutscher Naturschutztag DNT) took place in Munich from 26 July - 28 July 1925. Since 1957 the conference is held every two years.

  5. Hermann Joseph Müller (1890 - 1965) discovers X-rays to be agents of mutation.

  6. NABU (then the BfV) starts a campaign against destruction of the countryside through land clearance.