Someone will tell you simply -PMD More specifically, It's the result of damage from outside sources. Something hit the coin and pushed metal out of position.
I have seen alot of coins with similar PMD. Usually those will show some kind of marking, dent or sign of damage. I don't see any thing on the reverse of this coin.
It has been hit on the reverse at the N in ONE creating a bulge on the obverse. It looks like the dent on the reverse has been filled with "gunk".
I have removed the gunk on the reverse and still see no damage that would explain the bulge on the obverse. Here are some better pics.
OK, theory #2 - it's not PMD, it's a die break or chip, although most breaks either start at the rim, or on the lettering.
The depression on the reverse that is pushing that out on the obverse is located between the E Plu in E Pluribus and the O in One.
Still looks like there is an impact at that location. Another possibility is a bubble formed from heating that then collapsed during cooling.
Condor101 in my opinion you got it right. It is really easy to see that this coin has been in a fire from it's black color. when a nickel clad coin becomes extremely hot the layers will sometimes become seperated and air bubbles will get between the clad layers and swell out big or small areas. These have been sold on Ebay many times as error coins for big bucks to folks that don't know any better . The coin colectors are learning but by the hard way but still they are becoming more knowlegable.
