Over the last several hundred years, a number of bodies -- men, women, and children -- have been unearthed in the peat bogs of northwest Europe, specifically in Great Britain and Ireland, the Netherlands, the north of Germany, and Denmark. These have become known as "bog people", ranging in death date from 8000 B.C. to the early mediaeval period. No-one knows precisely how many bog bodies have been found, as many have disappeared or been reburied since their discovery, and there have likely been many more discoveries that went unreported.
Because of the tannins in the bog water, which tan the skin and preserve other tissues, most of the bodies are in extraordinarily good condition (thanks vuo!). They range from skeletons to complete bodies, some of them still fully-clothed with hair and skin intact, to limbs and heads without bodies, and bodies missing heads or limbs.
Originally, it was thought that these were the bodies of people who had become lost in the mist of the bogs and died of starvation, or of exposure during the winter. Then, researchers discovered something startling: many of the bodies showed signs that violence had been inflicted upon them shortly before their deaths. Some were even buried with the ropes used to garrotte* them still around their necks.
In 1879 not far from Ramten, Jutland in Denmark was unearthed the particularly well-preserved body of a woman, nicknamed Huldremose Woman. Her arms and legs showed signs that they had been hacked at repeatedly with swords, knives, or both. Her right arm had been torn from her body before she was left in the bog, sometime between 160 B.C. and 340 A.D.
In 1897 the body of a 16-year-old girl was removed from a small bog near Yde, Holland. Tightly wrapped around her throat was a woollen band, showing that she was strangled to death; there was a knife wound on her left clavicle. Near her body were remnants of a worn-out woollen cloak. Yde Girl was killed between 170 B.C. and 230 A.D.
Grauballe Man was discovered in 1852, also in Jutland. His throat had been cut, and his head and left leg had been struck repeatedly with a blunt, heavy object. Carbon dating places his death between 170 B.C. and 80 A.D.
These bog people and others had been murdered, or beaten to within an inch of their lives and dragged out into the fen to breathe their last. Why?
For ten thousand years following the last Ice Age, thick layers of peat formed in an area that stretched across much of northwest Europe. Peat bogs were marsh-like and mysterious, often shrouded in mist; people lived in fen villages on the high land that surrounded them. Travelling from village to village through the bogs, it was easy for one to become lost and drown, never to be heard from again. Therefore it is understandable that people would come to believe that they were inhabited by gods or spirits who controlled every aspect of their lives, from health to death and fate to crops and livestock.
Naturally, a good working relationship with these gods was desirable. Assurance of their favour could be brought about through sacrificial offerings; valuables were left in the bogs, many of which are still being discovered: pottery, weapons, jewellery. Sometimes, human sacrifices were made, perhaps in cases where a particular favour from the gods was required. These human sacrifices, then, are the bog people, who even today are still being unearthed.
Classical authors would hold it that during the Roman Iron Age, human sacrifices in northern Europe were offered not only to appease the gods, but also to celebrate such occurrences as military victories. Organised executions were also carried out as punishment for crimes and perceived imperfections, such as homosexuality.
Using skulls of bog bodies combined with measurements from modern populations as to the thickness of tissues covering the skull at various points, artists and anthropologists have been able to reconstruct in clay and wax what some of the bog people might have looked like just prior to their deaths. Wigs and skin pigmentation combine with the wax and clay base to create a striking image; these people who died thousands of years ago look almost contemporary.
A reconstruction of Yde Girl exists at Drents Musem, in Assen, the Netherlands. According to pathologists at Groningen University, she suffered from mild scoliosis; judging by slight deformations in her feet, she walked with a limp, her right foot twisted inward. This does not show in the reconstruction of her face. In keeping with her northern European heritage, her hair is long and blonde, hanging loosely around her face. She looks strangely and unsettlingly like Vermeer's "Girl with a Pearl Earring".
* No, really, it's spelt "garrotte". A cultural difference, I suppose.
Timeshredder says Of course, a bog could just be a really good place to dispose of your murder victims....
Sources:
The Mysterious Bogpeople: http://www.bogpeople.org/bog_uk/index.html
Archaeological Institute of America, "Bodies of the Bogs":
http://www.archaeology.org/online/features/bog/index.html
Civilization.ca - Media - The Mysterious Bog People: http://www.civilization.ca/media/docs/fsbog01e.html
And for further reading on this subject:
Through Nature to Eternity: The Bog Bodies of Northwest Europe, by Wijnand van der Sanden. Amsterdam: Batavian Lion International, 1996.
Bog Bodies: New Discoveries and New Perspectives,
edited by Richard Turner and Robert Scaife. London: British Museum Press, 1995.
The Bog Man and the Archaeology of People, by Don Brothwell. Cambridge: Harvard University Press, 1987. | 
