Plant examiners determined that the blast had started in a storage silo, essentially a huge pile of fertilizer. One minor problem they had faced from the start of Oppau was that granular fertilizer tended to cake in storage, to absorb water from the air and solidify into a rocklike mass. Workers at Oppau had long used explosive charges, small ones, to break up the huge piles for shipping. It was thought to be absolutely safe:
