1969 D Lincoln Cent Size Of Dime And Wafer Thin.

@mark nailed it. 100% acid bath.

I don't even think being in the ground for decades would corrode it that much (conjecture).

on both the acid damage assessment and that being burried would not cause it to dissolve like that.

It has been damaged with acid. PMD.

True that, or so it would seem.. However, in the Abstract of:

"The effect of soil conditions on the long-term corrosion of buried tin-bronzes and copper,"

R.F. Tylecote states:

"The object of this work was to relate the corrosion of tin-bronzes to the chemical condition of the soil in which they have been buried, most of them since the Bronze Age. Naturally, it was not easy to obtain recent hoards with their related soils, and considerable reliance had to be placed on recorded finds by taking recent soil samples from sites on which hoards were found over a hundred years earlier. Even so, with the exception of peat, a reasonable relationship appears to exist between the pH of the soil and the state of the metal. Acid soils are aggressive to metals and alkaline soils are benign. In no cases were sulphate reducing bacteria active in promoting corrosion. In the main, peat and peaty soils were benign in spite of their acidity, probably due to the protective action of polyphenols."
https://www.sciencedirect.com/scien...440379900189

Interesting..

Swamp

EDIT: For those wishing to delve a bit deeper, check this out:

"The Conservation of Early Post-Medieval Period Coins Found in Estonia"

Authors: Viljus, A. & Viljus, M., (2013). The Conservation of Early Post-Medieval Period Coins Found in Estonia . Journal of Conservation and Museum Studies . 10 ( 2 ) , pp . 30-44 . DOI: http://doi.org/10.5334/jcms.1021204

Abstract:

This article deals with archaeological find material with a low silver content and the problems of conserving such material. The aim of the research was to find the most suitable method for the conservation of poorly preserved early post-medieval period coins with varying composition. For this, first, the composition of both the metal and the corrosion products of the archaeological coins were analysed, after which comparative experiments of different cleaning methods were carried out in order to find out the least harmful and most efficient method. A test was also performed to determine the necessity and efficiency of stabilizing the surface of the coins after cleaning.
https://www.jcms-journal.com/articl...cms.1021204/

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Along with solid procedurals there are great photos of before / during / after..

Leaving no raised areas would mean the acid would only eat away at the devices (raised areas) untill it reached the surface of the coin - No, it eats away everything equally down at the same time - leaving the details to still be seen!

Whether dissolved by acid or electrolytic process, the highest rate of corrosion will occur at the rim, on the diametrical edges where both obverse and reverse surfaces intersect the rim. The corrosion is acting on both the rim, and on the obv/rev surfaces, so 2x the corrosion initially at those locations.