The nuclear bomb in Megaton didn't go off. Yet the city is built in an enormous crater, far larger than any conventional weapon I'm aware of can make. (In fact, I don't think the depth of the crater is characteristic of any explosion above the surface of the Earth.)
Since the warhead is largely intact, it seems like if it had a conventional implosive trigger, that also didn't go off.
Conveniently, the crater is also exactly the crater radius of the bomb if it does go off.
What caused the Megaton crater in the first place?