FYI https://www.coinworld.com/news/us-coins/2016/08/unexpected-bumps-on-coins-surfaces-vary.all.html 5. Plating blisters Plating blisters develop in the coin in the immediate aftermath of the strike, as gas expands between the core and poorly bonded plating. Among domestic coins, plating blisters are the exclusive province of copper-plated zinc cents. Blisters are generally small and subcircular, with a smooth surface and soft outline. They can occur anywhere on the field and design. The design continues uninterrupted as it crosses a blister. 6. Occluded gas bubbles Solely the province of solid-alloy coins, occluded gas bubbles form just beneath the surface and push up the overlying metal immediately after the strike. Like plating blisters, the surface is smooth and the edges soft. The design is uninterrupted. 7. Corrosion domes Contaminants trapped beneath or penetrating the surface of aluminum, plated zinc, and plated steel coins can react with surrounding metal to form an expanding front of spongy, corroded metal. The resulting solid dome will superficially resemble a hollow plating blister or occluded gas bubble. In many cases the corroded metal bursts through and may fall out, leaving a crater.
you have seen the beginnings of this - I think Paddyman originally posted this a bizillion posts ago about rotten Zincolns ...
