Can't see much in the pic, if the coin is in a flip you might try it out of the flip. The plastic always causes reflection and has particles on it that makes it tough to see through.
What you described sounds like it might be a bubble under the plating like you find on many zinc cents, the bubbles can be big and almost anyplace and even an unc coin can get oxidation or corrosion under a small bubble that makes it grow. _________________ Ed
