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?
Best Answer
The crater that the bomb left behind is not much larger than the bomb itself. If you look here, you'll see that the actual crater from the bomb is the greenish pool of water that sits directly above 'The Brass Lantern', written on the map. There is a slight upward slope immediately around that area as well, but nowhere near as severe as the crater itself.
Your confusion might have stemmed from the way the walls were built; but the walls were built around the bomb after it fell and failed to explode.