[RichardH]

The grand slam was a earthquake bomb and went a good 30 meters bellow the surface before detonation.
From movies showing it go off the explotion isnt that big at all but you have a great shockwave and a huge crater.
So instead of a atomic bomb like mushroom a more conventional explotion would be more true and have a rather bigger shockwave from it and a huge crater..
Seems daft with a atomic lookalike to a none atomic explotion if its whats getting modeled in my opinion.

Yeah. And also, the main reason why certain bombs create a mushroom cloud is because of the vacuum created when it detonates. (dust,debris etc rushing upwards to fill the vacuum and thus making a mushroom cloud.