Probably road rash damage which punctured the cladding exposing the zinc which is reacting to moisture in the air.
You're right, (that's why I should nap before posting...) It's the impurities that causes the corrosion (reaction): http://www.keytometals.com/Article40.htm
Wonder if it could be a mint error (a totally worthless mint error) If ther were impurities between the copper and the zinc that acted as a electrolyte the penny would become a battery.
think it went through a wash cycle and the detergent got under the plating and ate away the zinc. I get them on occasion that eaten away by nearly 40%. A friend calls it "coin cancer". I usually just recycle them.
I think mostly it's caused by household chemicals or detergents reacting with the zinc. I caused one to rot away almost completely by soaking it in red wine cider. The reason I think it is common substances reacting with them, is that they are fairly common.
salted winter road rash? or someone stuck it to a car battery to keep the terminals from corroding? actually i just noticed you location. someone dropped it while sticking bottles upside down in the salt fields?
Actually, it's used as a sacrificial anode. Zinc is more reactive than iron or copper; but, when it's bound to one of those metals -- that is, in good electrical contact with it -- it gets attacked before the less-active metal. So, if you've got a part made of galvanized steel, the zinc will slowly corrode off it. But you don't notice unsightly deposits, because the products of zinc corrosion are colorless and water-soluble; and, until most or all of the zinc is gone, the iron remains intact. On a copper-coated zinc penny, if you puncture the copper layer, the zinc immediately starts getting attacked. In fact, I imagine it gets attacked faster than the coating on galvanized steel, because the difference in "electronegativity" is greater with copper than with iron. Any acid, or even a strong base, will speed things along considerably. (Zinc oxide is "amphoteric", meaning it can react either as a base or an acid.) So, yeah -- punch through the copper coating, and you get pits that grow rapidly (by coin-decay standards, anyhow). Maybe BadThad can fact-check me on this...?
on the i 80 between salt lake city and reno, there are salt flats (fields?). anyway, for some reason, people stick bottles in the salt (upside down) and some spell their names with the bottles. you can actually make out some of the bottle shapes on google maps satellite view (then street view) in bonneville salt flats state park, (a little over an hour away from you.)
