Where was the largest known explosion prior to the Hiroshima bombing?

A French Vessel called the Mount-Blanc entered the narrows on December 6th, carrying over 2300 tons of picric acid, which NASA pointed out is "more powerful and volatile by weight than TNT". In addition, it carried 250 tons of TNT, almost the same weight of benzene, over 60 tons of nitrocellulose, plus about 300 rounds for the deck guns mounted on the ship.

around 10,000 times that of the atomic bomb dropped on Hiroshima in August 1945.

Halifax Harbour remains unchallenged in overall magnitude as long as five criteria are considered together: number of casualties, force of blast, radius of devastation, quantity of explosive material, and total value of property destroyed.