Hiroshima and Nagasaki didn’t, but other detonations did, like Castle Bravo.
In: Chemistry
To dig a crater you more or less have to have the fireball actually in contact with the ground. A weapon detonated high enough that the fireball does not contact the ground will not leave a crater. The blast wave is powerful enough to knock down houses and buildings, but that is still not strong enough to excavate a hole by itself. The fireball, however, can vaporize the ground, and has immense pressures and temperatures, and that is enough.
Hiroshima and Nagasaki were high airbursts; the fireballs never came even close to the ground. Bravo was a huge device detonated on the ground itself. Trinity was a bomb the size of Hiroshima and Nagasaki, detonated on a not-very-high tower, and did leave a modest crater (which was about the size of the fireball).
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