That one would really get me thinking! I could easily be a unique looking gas bubble as Chris said, but any chance this is a dropped letter error from the O on the reverse? I guess that would be incuse not raised so that can't be it. Cool find no matter what.
Zinc corroding below the plating. If the bubble gets hit by another coin, it'll partly depress.
Much too well defined and deliberate to be the remnant of a gas bubble IMHO. This is not like any imploded bubble that I have seen (and I've seen a few). Reminds me more of the mystery circle on reverse of some LMCs.