This big blob below the MM does not look anything like a bubble. It may be but, if it's not what else could it be?
It looks like a chipped die although it's odd that a chip would break out and leave a ridge between it and another incuse part (the mm) of the die. And it looks too smooth and even to be a chip. More likely it's a strike through debris of some type.
its zinc rot, the plating split around the mint mark exposing the zinc core and allowing it to start corroding under the plating...just like rust forming under the paint of a car...
i have so many like yours with planets,arrows,some even resemble a toilet seat,i keep them just for fun yours seems to fall in the sane category as mine cute but worth a penny only ....
Thanks for the link Paddyman. I have a few cool bubble ones too. One where it looks like a booger and another like arrows through the head. I didn't think that one was a bubble but it has to be.