I thought about what you said and if the nuke went off underground it would first go straight up since it would blow the ground above it away first "the ground next to the nuke underground would just compact since it's got more ground beside it but the above stuff would pop like a zit". So you'd get an almost totally upwards blast like this that would spread as the ground on the side of the opening would open later "second or so to fully open" then the blast would spread horizontally more after the initial blast that shows the mushroom cloud. I'm assuming to look like the image it might be a 100ft down or so to show like a tube with the mushroom as it funnels through the passage to the surface.