Let's try galvanic action along with sesmic activity, weather, erosion and countless other things such as specific gravity.
Earth grounding of everything which either uses or produces a electric current is necessary to prevent galvanic action from destroying the components of the electronic device.
Anyone who has ever metal detected around the buried cables of a power utility's transmission station or in the vicinity of a cell phone tower with bad grounding has experienced the audible consequences of galvanic activity in their head phones that is talikg place in the ground below their feet.
In case you aren't tracking here, the hulls of steel boats used in salt water must have sacrificial lead ingots attached to moving parts such as propeller shafts and other moving parts to protect them from destruction as the electrical current produced by the moving shaft is transferred to the salt water ground using the steel hull as the conductor.
For similar reasons the aluminum hot water heater of your RV, when wired for AC current, and the copper core of your cars radiator must be grounded to the steel frame or they too will be destroyed by galvanic action.
The exact principle of why this occurs is to complex and lengthy to fully explain here but it can be found nearly anywhere electronic theorys are discussed.
Even the moist air passing over the wing of a aluminum airplane produces a sufficient amount of galvanic action that aluminum areas surrounding the steel screws and rivets must be inspected and replaced regularly because the effects of galvanic activity will change its physical properties, and eventually destroy it.
Trashed zincolns are a main find, and many aren't so old that soil acidity can be held accountable for their miserable condition.
But the flow of electrical energy, produced both naturally by the difference in the minerals found in the soil of the area as well as man made electrical devices using earth ground can.
Zincolns are more prone to galvanic action than coins made of silver, gold or platinum because both copper and zinc are excellent conductors of electricity...check what your car battery is made of.....and both dissimilar metals are contained in a compact package which is already assembled to produce a electrical field of its own.
Disagee if you want, then go to the garage and count wealth of zincolns you have and then compare it to the number of silver, gold and platinum coins you have found at the same depth.
Oh yea, for the die hards which still believe that metal can't be moved by electrical current, go watch a arc welder as it moves molten metal from one surface to another,
Just a thought............
AT Pro/GPP/Fiskars Diggers