Wasn’t an ammonium nitrate explosion (military high explosive being shipped to Europe) but the Halifax ship explosion in 1917 killed 2,000 people (including some from a tsunami in the harbor) and was the largest explosion ever prior to the Trinity nuclear test. My sister does our genealogy but I think I had a distant relative killed in the blast:
http://en.wikipedia.org/wiki/Halifax_Explosion
Terrible events then....