IIRC The largest non nuclear explosion was the Halifax explosion on 6 December 1917 in Canada during WW1.
Two ships - the IMO and the Mont Blanc collided in Halifax Bay. One was carrying huge amounts of explosives. The crew quickly abandoned ship and rowed to shore. Once there they ran through a crowd of curious onlookers and headed for the hills. "What's that all about?"
The resulting explosion was equivalent to 2.9 kilotons of TNT (about 20% of Hiroshima bomb), killed 2,000 people and severely damaged Halifax.