We can only guess why these ancient people, whose perfectly preserved bodies are found in swamps throughout Northern Europe, were so brutally killed. A BBC Travel correspondent is trying to penetrate the mystery.
Our train, heading from Hamburg to Denmark, slowly passed green fields and birch groves flooded with water, and outside the window appeared another swamp, shaded by willows and overgrown with blue-green algae and duckweed.
Even sitting in the carriage, I felt how dark and quiet they are, these peaceful bogs - this is how, I think, a pond not far from here, in Helsingor, where Hamlet's unfortunate beloved Ophelia found her last refuge, should have looked like to me.
The train entered the kingdom of the swamp people.
Swamp people are called our ancestors who lived two thousand years ago. Their bodies are found in bogs throughout Northern Europe - from Ireland to Poland.
Many modern archaeologists believe that these people, who lived in the Iron Age, were killed and carefully lowered into the swamp as a ritual sacrifice to the gods.
Some scholars claim that they were criminals, foreigners or vagabonds.
Denmark occupies one of the first places in the world in terms of the number of marshes and marsh people, many of which have been perfectly preserved for many centuries, as if preserved in the acids released by peat moss (sphagnum), the living "foundation" of these swamps.
Most of the bodies were found by accident during peat mining in 1800-1960, when it was still used as fuel in Denmark.
Autopsies carried out with the help of the latest advances in forensic medicine showed that almost all of them - both men and women - died a violent death: some had finely cut loops around their necks, some had a terrible gaping throat.
Since very little is known about Denmark during the Iron Age - at that time there was no written language here, and almost nothing has survived from Roman and Greek sources to this day - we can only guess who they were and why they were killed.
However, since in most cases the bodies of the dead were burned at that time, we know that these unfortunates suffered a different end than their contemporaries.
I wanted to look with my own eyes at the swamp people in order to try to understand the mysterious world from which they come.
The first stop on my way was the small town of Vejle in the southeast of Jutland, 240 kilometers west of Copenhagen. Vejle is inhabited by about 100 thousand people.
This luxurious rolling landscape is not typical of lowland Denmark, with roads winding through gentle slopes of cultivated hills and glacial valleys dotted with water-filled hollows and forest marshes surrounded by pink orchids and tough brown cattails.
I came here to meet Mas Raun, the chief archaeologist of the Vejle Museum and curator of an amazing collection of artifacts, including Roman coins, engraved swords, and swastika brooches (in fact, this ancient symbol existed even before his adopted by the Nazis). All this was found in the swamps.
From a dark room in the back of the museum, a mournful hum of a deer antler was heard - now my contemporary was blowing into it, but in the Iron Age in Denmark this sound often served as a distress signal. I heeded his call and hurried inside.
In the darkness, in an open glass sarcophagus, lay the fragile, thin-skinned body of a woman from Haraldsker, whose ash-gray face was frozen in an expression of deep shock.
She didn’t look as peaceful as the swamp people I’ve seen in the books, and it gave me goosebumps - it felt like I was intruding on something personal.
“In 1835, when peat miners found her, she was mistaken for the Viking queen Gunhilda, who lived in the 10th century, who, according to the Jomsvikings saga, was drowned by her husband Harald Bluetooth,” Ravn told me, scratching his beard and glancing curiously on the body.
“But it’s not, and now, thanks to radiocarbon analysis, we know that it is about 2,200 years old.”
A woman from Haraldsker was found nude at the bottom of a swamp, to which she was crushed by tree branches - probably after her death.
Her clothes lay next to her body. Judging by the marks on her neck, the woman was strangled.
In the course of an additional postmortem examination, the contents of the stomach at the time of death were examined: it contained unpeeled millet and blackberries - an atypical menu of the last meal for a society that ate mainly meat.
“We are currently doing an isotope analysis of her hair and applying a new DNA test based on the extraction of a DNA sample from the inner ear. We hope to get results soon and learn more about it."
Raun and I drove to Haraldsker (Harald's Marsh), where the woman was found.
This unremarkable place is located 10 kilometers west of the museum. Like the swamps I had seen from the train window, it was covered with bright green duckweed and surrounded by a dense ring of trees, under which twisted mushrooms with burgundy caps and fiery red berries dazzled in patches of sunlight.
There is something witchcraft and otherworldly in these swamps - here it becomes clear why they were chosen a long time ago to make sacrifices and why they still retain their inexplicable appeal to this day.
Further on my way lay Aarhus - the second largest city in Denmark. I came to see the amazing exhibits on display at the new Moesgård Museum, which houses one of the finest Iron Age exhibitions in Europe.
The main "star" of the exhibition is a man from Groboll. He was found in 1952 sitting in a naturally relaxed posture, as if he was doing yoga, and perfectly preserved: his feet and skin were left practically intact, and his face with sharp features and a button-like nose looks almost the same as in life.
“Like most swamp dead, his hair and skin were dyed red through a chemical process called the Maillard reaction,” explained archaeologist and exhibition manager Pauline Asingh. "Handsome man!"
However, the serene expression on the face of the man from Groboll does not fit in any way with his terrible death.
“They put him on his knees and, standing behind, slit his throat from ear to ear. Then they very carefully lowered them into the swamp,”Asingh continued her story.
"It may seem cruel and reckless to us, but sacrifices were an important part of the cultural life of people in those days."
Asingh then took me to another exhibit, this time dedicated to the dogs killed in the swamps.
In 2015, 13 dogs were found in the Sködstrup bog near Aarhus, sacrificed around 250 AD, which means that the ritual sacrifices were not only human.
Moreover, this exhibition features several touching series of drawings showing that deep affection was behind these sometimes brutal murders.
One of them depicts a young girl decorating her dog's neck with a wreath of flowers before being executed.
A visit to the Moesgård Museum made me think: no matter how we wanted to simplify the past, our ancestors were people, not artifacts, and each had their own difficult life story.
The last stop on my way was the small town of Silkeborg, 44 kilometers west of Aarhus.
Here, in the light yellow building of the Silkeborg Museum, there is a small but impressive collection of exhibits dedicated to swamp people, and it also houses one of the best-preserved examples.
The man from Tollund, whose age is about 2,400 years, has survived to this day safe and sound - so much so that in the 1950s, when his body was discovered, local authorities took him for the corpse of a recently missing boy.
Like some of his companions in misfortune, this swamp man was hanged - an ingeniously woven noose still hung around his neck. But both his long nose and his smooth forehead were not touched by decay, and his full lips folded into a mysterious half-smile.
Her lovely 90 cm long red braid is tucked into an intricate knot.
In the next room, a woman from Elling was waiting for me, found only forty meters from the man from Tollund and who died at about the same time.
Apparently, she was also hanged. Her lovely 90 cm long red braid is tucked into an intricate knot.
Museum archaeologist Ole Nilsson took me to Bjellskoudal, where both bodies were found, a wide swamp about 15 kilometers from the museum.
Since then, the site has been designated a nature reserve, equipped with wooden decks and marked trails.
During our quick walk, a light fog hung over the lake-like swamp, and along the way we now and then met big blue herons, mallards, purple flowers, and, of course, there was porous sphagnum moss everywhere.
Stopping to admire the swamp, I wondered what other secrets its dark depths hold.
The swamp was squelching - slowly and greedily, as if recalling its eternal and terrible power over what had fallen into its swamp for millennia.