In 1921, a huge explosion at a fertilizer factory at Oppau, Germany, caused 500 deaths. Destruction at the site included hundreds of neighboring houses. The fertilizer made there since 1917 was a 50/50 mix of ammonium nitrate and ammonium sulphate. Storage silos reached up to a hundred feet high. This fertilizer had a tendency to form solid clumps. It had become routine to break it up ammonium sulphate with bulldozers and dynamite. The mixed fertilizer was not regarded as explosive because it was difficult to initiate and not experienced before in such circumstances. Dynamite used this time caused 4,500 tons of fertilizer to explode, creating a crater 250-ft diam. and 50-ft deep.«