They would not detonate at ground level to maximize destructive potential. To contrast, the US actually has different bombs that penetrate into the ground before detonating in order to effectively destroy buried bunkers.
For example, I believe the bombs dropped on Hiroshima and Nagasaki detonated at about 500m above the ground.
For example, I believe the bombs dropped on Hiroshima and Nagasaki detonated at about 500m above the ground.