In 1921 a 4,500 metric ton stockpile of ammonium nitrate and ammonium sulfate exploded at a chemical plant in Oppau, Germany.
The way I remember the story is that this was a huge pile sitting outside, byproduct from some other process. Over time, exposure to the elements had caused the pile to fuse into a rock hard mass. So, when they decided to clean up the pile, to break up the fused mass they decided to blast. They drilled holes into the hardened mass, inserted dynamite and set off the dynamite hoping to break the pile into smaller pieces.
When was the Port Arthur, Texas explosion? This involved a freight ship filled with amonium nitrate that caught fire. Even though it was taken a good distance from port before the explosion it still caused great damage to the town.