The Frenchman Gustave Dore's illustrations of Dante's Inferno date to 1857, in one of which he shows damned traitors in the ninth circle of hell, frozen in the mythological river Cocytus around Hades, imagined rather as a lake shallow enough for his scene, and reminiscent of a bog, what the English in the mid-18th century called a tarn.
Tollund Man is a famous example of a bog body, dating from ~400 BC, found well-preserved in a Danish peat bog in 1950. He died by hanging, as did a number of the many bog bodies which have been discovered in Northern Europe, found as they were with the nooses still around their necks. Some of these bodies also show other signs of violence.
You might say something rotten was found in Denmark.
Bogs, swamps, and marshes have also been associated for centuries with ghosts, fairies, and spirits of one kind or another, perhaps on the strength of the misinterpretation of the natural phenomenon of the occasional ghost light seen at night in bogs. The elusive lights are due to ignition of gases from the decay of organic matter. The ghost light is known as the ignis fatuus or foolish flame, among many other names used for the phenomenon around the world.
The word tarnation is first used in literature as far as we know from the pen of the Bostonian Royall Tyler in his 1787 play The Contrast. Etymologists plausibly style its origin merely as a North American euphemism for damnation and do not associate it with tarns.
Perhaps it's more complicated, and more simple, than that.
