Cast into northern European wetlands, bog bodies have long appeared as opaque to archaeologists as their dark and watery graves. But new clues are coming in the centuries-old mystery of their origins.

Over 500 Iron Age bog bodies and skeletons dating to between 800 B.C. and A.D. 200 have been discovered in Denmark alone, with more unearthed in Germany, the Netherlands, the United Kingdom, and Ireland. (Read "Tales From the Bog" in National Geographic magazine.)

Much of the bodies' skin, hair, clothes, and stomach contents have been remarkably well preserved, thanks to the acidic, oxygen-poor conditions of peat bogs, which are made up of accumulated layers of dead moss.

Tollund Man, for example, found in 1950 on Denmark's Jutland Peninsula and perhaps the most famous bog body in the world, still "has this three-day beard—you feel he will open his eyes and talk to you. It's something that not even Tutankhamun could make you feel," said Karin Margarita Frei, a research scientist who studies bog bodies at the National Museum of Denmark.

In Denmark, about 30 of these naturally mummified corpses are housed in museums, where scientists have worked for decades to figure out who these people were and why they died.

Because some bear horrific wounds, such as slashed throats, and were buried instead of cremated like most others in their communities, scientists have suggested the bodies had been sacrificed as criminals, slaves, or simply commoners. The Roman historian Tacitus started this idea in the first century A.D. by suggesting they were deserters and criminals. (See National Geographic's pictures of bog bodies.)

But ongoing research is uncovering an entirely new dimension: When alive, these people of the bog may have instead been special members of their villages, which in the early Iron Age were loosely scattered across Denmark.

New chemical analyses applied to two of the Danish bog bodies, Huldremose Woman and Haraldskær Woman, show that they had traveled long distances before their deaths. What's more, some of their clothing had been made in foreign lands and was more elaborate than previously thought.

"You sacrifice something that is meaningful and has a lot of value. So maybe people who [had] traveled had a lot of value," Frei told National Geographic at the Euroscience Open Forum in Copenhagen in June.

View Images Analysis of Huldremose Woman's leather cape and woolen scarf and skirt show that the clothing had been made outside of Denmark. Photograph courtesy of The National Museum of Denmark

Supernatural Portal?

For Europeans dating as far back as the Neolithic period 6,000 years ago, bogs were both resources and possibly ominous supernatural portals, according to Ulla Mannering, an expert in ancient textiles at the National Museum of Denmark.

The bogs' peat, which could be burned for heating homes, was valuable in tree-scarce Denmark, and an ore called bog iron was made into tools and weapons.

Among prehistoric people, "when you take things, you also offer things," said Mannering.

This may be why the Danish villagers would deposit "gifts" of clothes, old shoes, slaughtered animals, battered weapons, and, for a period of 500 years, people into the black abyss of the bogs. (Related: "Medieval Christian Book Discovered in Ireland Bog.")

Danish Iron Age cultures left no written records, so their religious beliefs are unknown, Mannering noted.

"Very Fine Lady"

When peat harvesters began accidentally unearthing bog bodies in the mid to late 1800s, many were discovered without clothing, solidifying the view that they had been simple people, Frei said. (Watch a National Geographic Channel video about bog mummies.)

Tollund Man, for instance, was found with a belt but no clothes. "It doesn't make sense to be naked and have a belt," Frei pointed out.

Frei wondered, then, if some of the bog bodies' clothing had dissolved in the bogs over the centuries. So she decided to examine Huldremose Woman, a mummy discovered in 1879 wearing a checkered skirt and scarf, both made of sheep's wool, and two leather capes.

Using microscopes, she discovered that tiny plant fibers were stuck to the 2,300-year-old woman's skin—remnants of ancient underwear, which later analyses revealed were likely made of flax.

Next, Frei performed a first-of-its-kind analysis of the strontium isotope contained in the flax and in the wool from the skirt and scarf.

Researchers analyzed the isotopes, or different varieties, of atoms in the strontium preserved in the flax and wool fibers. These atoms provide a chemical insight into the geology of the region where the plant and sheep lived.

The results show that the plant fibers taken from threads of the underwear grew on terrains geologically older than those of Denmark—those typical of northern Scandinavia, such as Norway or Sweden—suggesting that Huldremose Woman may have come from somewhere else, according to research published in 2009 in the Journal of Archaeological Science.

Frei also did an analysis of strontium isotopes in Huldremose Woman's skin. Humans absorb strontium through food and water, and it's especially prevalent in our teeth and bones—though many bog bodies are found without teeth and bones because of the acidic conditions.

View Images Haraldskær Woman, who is kept at Denmark's Vejle Museum, was first thought to be a Norwegian queen. Photograph courtesy of Vejle Museum 2013

The research revealed that Huldremose Woman's body contained strontium atoms from locales outside Denmark—showing she had traveled abroad before she ended up in the bog. (See "Pictures: Ancient Bog Girl's Face Reconstructed.")

Another study published in 2009 by Mannering revealed that Huldremose Woman's woolen garments—turned brown by the bog—were originally blue and red: Dyed clothing is a sign of wealth, she says. Mannering and colleagues also found a ridge in Huldremose Woman's finger that may have indicated it once bore a gold ring before it disintegrated in the bog.

"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 said.

"It had been overlooked that there is such a wealth of resource in these textiles and skin," Mannering added.

What's more, the research shows that early Iron Age cultures of the centuries just before and after Christ were more interconnected than thought, Frei said.

For instance, wool and plant fibers were either traded or brought as raw materials for textiles more commonly and over longer distances than previously assumed, according to Frei's research.

"This was seen as a period where society was very closed—not much trade with the outside world," she said. "Suddenly we can see, yes, there was."

Lotte Hedeager, an expert in Iron Age archaeology at the University of Oslo in Norway, agreed, noting that "these results require a rethinking" of the communication and trade networks among northern European cultures in the early Iron Age.

The Hair Has It

Building on their discoveries with Huldremose Woman, Frei and colleagues wanted to see if other bodies were outsiders.

So they turned to Haraldskær Woman, a bog body housed at the Vejle Museum in Denmark, who was found in 1835 and originally thought to be the Norwegian Queen Gunhild. (Also see "'Frankenstein' Bog Mummies Discovered in Scotland.")

New developments in strontium isotope tracing technology make it possible to detect strontium isotopes in human hair, which can show where a person lived over the last few years of his or her life. Since hair grows slowly, analyzing strontium atoms at the root of someone's hair versus the bottom of their hair may reveal geographic movements.

The longer the hair, the longer the record of their movements—which makes Haraldskær Woman, with her 20-inch-long locks (50 centimeters), a perfect subject.

Excitingly, preliminary results of the analysis, still unpublished, mirror the findings for Huldremose Woman—Haraldskær Woman had lived elsewhere before her death. The scientists are also examining her clothing, which may have been produced in another location.

"DNA cannot tell you that—it can tell you your genetic makeup, but [not] where you were born, where you had your childhood, where you had the last years of your life," Frei said.

Frei and colleagues are now running strontium isotopic analyses on the skin of Tollund Man to see where he'd been before his death.

View Images A small part of the bog remains where Tollund Man was found on Denmark's Jutland Peninsula. Photograph by Robert Clark, National Geographic

Geographic Outsiders

Other experts agree that the research indicates the bog bodies were people considered unique in their villages.

To Heather Gill-Frerking, a mummy researcher for the museum-exhibit company American Exhibitions, the new findings are "really nice evidence" for her theory that bog bodies were what she calls "geographic outsiders"—people who may have married into Danish communities, or had been doing work or apprenticeships abroad.

Gill-Frerking has suggested for years that bog bodies weren't the results of some religious rite, but were instead foreigners placed into the bog.

These people may not have been cremated like everyone else because they hadn't yet assimilated into their communities, or perhaps because the communities weren't aware of the dead person's burial customs. (She said it's likely some of the bog bodies were buried after they'd died of natural causes.)

"I'm a big believer of multiple interpretations of bog bodies, not just [that they were] rituals," she said.

If Frei continues to find that those buried in the bog had traveled before their deaths, "we'd need to look hard at the ritual theory and look at bodies as individuals."

"Secret of the Bogs"

Niels Lynnerup, a forensic anthropologist at the University of Copenhagen who has studied bog bodies, believes that they were sacrificed—but the enigma, he said, revolves around why.

The new discoveries that at least one of the bodies may have been that of a foreigner, he says, "certainly adds to that discussion of, 'Who were the people who were sacrificed?'" (See "Murdered 'Bog Men' Found With Hair Gel, Manicured Nails.")

For instance, Lynnerup suggested that perhaps they had a special status because they came from abroad or were hostages of raiding parties into other areas.

It's also possible that, as was the case with some Inca sacrifices, the person considered it an honor to be chosen and voluntarily went to the bog.

"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."

Overall, the archaeologists acknowledged it's likely there will always be more questions than answers when it comes to mysterious bog bodies.

Added the University of Oslo's Hedeager: "We will never be able to uncover the perception of life and death of those individuals 2,000 years ago.

"That remains a true secret of the bogs."