The largest man-made explosion ever was the triple stage thermonuclear bomb Tsar Bomba, tested by the Soviet Union on October 30, 1961. Generally rated at about 50 megatons, it was actually “dampened” by using a lead casing instead of a uranium casing, which likely would have doubled its yield, at the cost of horrendous downwind fallout.
The fireball is about 5 MILES across.
Obviously, no conventional explosives can rival thermonuclear weapons for their explosive power. Indeed, the largest deliverable conventional weapons pale in comparison. The US MOAB is a paltry 8-1/2 tons of explosives. The Russian Father of All Bombs carries a bit less, but is actually more powerful:
But that doesn’t mean man hasn’t been responsible, either deliberately or by accident, for some truly massive explosions.
Sean Linnane has compiled a list of some of history’s most massive, or most deadly explosions.
There have been many extremely large explosions, accidental and intentional, caused by modern high explosives, boiling liquid expanding vapor explosions (BLEVEs), older explosives such as gunpowder, volatile petroleum-based fuels such as gasoline and other chemical reactions. This list contains the largest known examples, sorted by date. An unambiguous ranking in order of severity is not possible; a 1994 study by historian Jay White of 130 large explosions suggested that they need to be ranked by an overall effect of power, quantity, radius, loss of life and property destruction, but concluded that such rankings are difficult to assess.
The weight of an explosive does not directly correlate with the energy or destructive impact of an explosion, as these can depend upon many other factors such as containment, proximity, purity, preheating, and external oxygenation (in the case of thermobaric weapons, gas leaks and BLEVEs).
It’s worth scrolling through the list. Of particular interest is the large number of Royal Navy capital ships that suffered catastrophic explosions due to aged cordite.
Here are a couple of my favorites.
The Toronto propane explosion.
(Somewhat NSFW for adult language)
And of course, the British Heligoland explosion
XBradTC 
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