It's the future. The future of every doomed-from-birth Lincoln cent with the misfortune to have been struck on a copper-coated zinc planchet.
I said a long time ago that the zinc cents would suffer, even in their slabs. I have a couple high grade 2009 P error cents that are toning terribly in their ANACS slabs. I can't get rid of them fast enough. And yes, they are stored in a proper environment.
That's what it is. Yes. I have. While I was active duty serving onboard the USS Midway back in the late 80's, our head was messed up. A group of HT's came in and removed a pipe that serviced a bank of urinals. After removing the pipe, they emptied out the elbow portion of the pipe and out came a bunch of decayed pennies. Some looked just like this one, others were even worse, just depending on how long they had been in there.