The plting on the coin was split during the strike. When the zinc got air, it started to break the planchet down. Creating a crater under the edge of the mint mark on the SW. This was common on the coins from the 1980's that were RPMs, but it is still happening when the mint mark punch is part of the design. Probably if you look on other parts of the coin you might see on the reverse areas around the tops of the devices and areas outside the Memorial where you can see this happening as well. This is caused when the coin was struck with too much pressure.
Edited by coop
10/06/2016 08:54 am