In Denmark alone, more than 500 bodies and skeletons were buried in the peat bogs between 800 B.C. and A.D. 200. Thanks to lack of oxygen and the anti-microbial properties of peat moss, their mummified corpses have been extraordinarily well preserved, including discernible facial features, fingerprints, hair, nails and other traits.

As Iron Age Europeans left no written records about their customs or their religious rituals, researchers can only speculate as to why the bodies were buried in the bogs instead of cremated, which was the general practice at the time. Because many of the bog bodies bear signs of gruesome violence, such as slashed throats, one long-standing theory is that they may have been human sacrifices. One relatively contemporary account that supports this theory belonged to the Roman historian Tacitus, who recorded the accounts of Romans who traveled to Northern Europe to trade goods in the first century A.D. He wrote that the northern tribes they interacted with practiced human sacrifice, lowering criminals, deserters and other “unnaturally immoral” people into “filthy swamps.”

As reported in National Geographic, more recent research has opened a new and fascinating window onto the mystery of the bog bodies. While many of the bodies were found without clothing on, supporting the idea that they were simple commoners, the 2,300-year-old Huldremose Woman (discovered in 1879) wore a skirt and scarf made of sheep’s wool, as well as two leather capes. When Karin Margarita Frei and her colleagues at the National Museum of Denmark examined the body under microscopes, they discovered tiny plant fibers stuck to the skin. These remnants of ancient undergarments–likely made of flax– were in turn analyzed. By tracing the isotopes in strontium, a chemical element in the flax fibers, researchers found that the plant likely originated in a terrain geologically older than that of Denmark–someplace more similar to northern Scandinavia, such as Norway and Sweden. They also traced isotopes in the strontium in Huldremose Woman’s skin to outside Denmark, supporting the hypothesis that she traveled before she died. “At first we thought this must be a witch–now we think she’s a very fine lady with expensive jewelry and expensive clothes and underwear,” Frei explained.

This research (published in 2009) has now been bolstered by a brand new analysis of Haraldskær Woman. Found in 1835, the mummified corpse was one of the first bog bodies scientists studied. It was originally thought to belong to the Norwegian Queen Gunhild (though this was later disproved), and is now housed at the Vejle Museum in Denmark. With new developments in strontium isotope tracing technology, scientists have now analyzed the 20-inch (50-centimeter) long locks of hair preserved on the skull of Haraldskær Woman, and their as-yet-unpublished findings show that she, like Huldremose Woman, lived somewhere else before her death.

The new findings appear to support the hypothesis that, rather than criminals, slaves or commoners, the bog bodies may have been people singled out as important in their villages. Heather Gill-Frerking, a mummy researcher for the Florida-based museum-exhibit company American Exhibitions, has long argued that the bog bodies were “geographic outsiders,” who may not have been cremated because they hadn’t been assimilated into local customs. Gill-Frerking also suggests that some may have perished due to natural causes, rather than ritual sacrifice.

Niels Lynnerup of the University of Copenhagen, says the fact that some of the bog bodies traveled before their deaths does not rule out the ritual sacrifice theory, but adds a layer of complexity to the discussion. Lynnerup argues that it’s possible that the bog bodies could have been hostages from raiding parties chosen for sacrifice, or that–as was the case for some human sacrifices in the Inca culture–they could have gone willingly into the bog, as it was an honor to be chosen to sacrifice oneself to the gods. In any case, he says, “Having the additional information that at least one of them was not local is terribly important, and [it] will be highly interesting to see if this pattern goes on.”