Not a die crack. If it was it would have to start at the rim and proceed inwards. As others have said, zinc plating blister. I would think that the zinc plate had a minor burn blister on the sheet prior to stamping of planchets, then when the coin was Stamped, the blister broke then flattened. Just as if you got a burn blister on your skin, then lance it with a needle, skin doesn't all go back to normal texture., at least not for the moment.