Replies

The worst harbor explosion of all time was Halifax, Nova Scotia, in 1917...

http://museum.gov.ns.ca/mma/AtoZ/HalExpl.html

"At 7.30 a.m. on December 6, the French ship Mont Blanc left her anchorage outside the mouth of the harbour to join a convoy gathering in Bedford Basin. She was loaded with 2,300 tons of wet and dry picric acid, 200 tons of TNT, 10 tons of gun cotton and 35 tons of benzol: a highly explosive mixture. At the same time the Norwegian vessel Imo, in ballast, set off from the Basin bound for New York to pick up a cargo of relief supplies for Belgium. At the entrance to the Narrows, after a series of ill-judged manoeuvres, the Imo struck the Mont Blanc on the bow. Although the collision was not severe, fire immediately broke out on board the Mont Blanc. The captain, pilot and crew, expecting the ship to blow up immediately, launched the lifeboats and took refuge on the Dartmouth shore.

Just before 9.05 a.m., the Mont Blanc exploded. Not one piece of her remained beside the dock where she had finished her voyage. Fragments rained on the surrounding area, crashing through buildings with enough force to embed them where they landed.....

1,630 homes were completely destroyed, many by fires that quickly spread following the explosion; 12,000 houses were damaged; 6,000 people were left without shelter. Hardly a pane of glass in Halifax and Dartmouth was left intact.
The death toll rose to just over 1,900. About 250 bodies were never identified; many victims were never found. Twenty-five limbs had to be amputated; more than 250 eyes had to be removed; 37 people were left completely blind. Hospitals treated well over 4,000 cases, and private doctors hundreds more."

It was apparently the largest man-made explosion prior to the Trinity test.

I've seen assorted mention of the blowing up of some rock that was a hazard to navigation in British Columbia as the largest non-nuclear explosion ever, but I think that was later.