At least one thing appears nearly certain about the ancient twentysomething: He was the victim of a carefully staged sacrifice.
Recent studies have revealed that Lindow Man's head had been violently smashed and his neck had been strangled and slashed.
Druid Fountain of Blood
"You've got a rope tightened round his neck, and at the moment where the neck was constricted, the throat was cut, which would cause an enormous fountain of blood to rise up," said archaeologist Miranda Aldhouse-Green, an archaeologist at Cardiff University in Wales and an expert on the Druids.
Another clue lay inside the body's well-preserved gut: pollen grains from mistletoe, a plant that was sacred to the Druids. (Romans wrote that Druids cut mistletoe from trees with golden sickles.)
Lindow Man's death is dated to around A.D. 60, when the Romans launched a new offensive in the island of Great Britain, currently part of the United Kingdom.
He may have been sacrificed to persuade the Celtic gods to halt the Roman advance, Aldhouse-Green said.
"Something had to be done to stop them in their tracks," she said in the documentary. "And what better way than sacrificing a high-status nobleman?"
The idea jibes with something Julius Caesar wrote: In times of danger, the Celts believed that "unless the life of a man be offered, the mind of immortal gods will not favor them."
VIDEO: Lindow Man, Druid Sacrifice? (With Dramatizations)
Mass Druid Sacrifice?
Other grisly clues come from a cave in Alveston, England.
Skeletons belonging to as many as 150 people and dating back to about the time of the Roman conquest were discovered in 2000.
Druids may have killed the victims—who show evidence of skull-splitting blows—in a single event. It may have been the Roman invasion itself that escalated the Druids' ritualized slaughter, researchers say.
Mark Horton, an archaeologist at the University of Bristol, thinks the pile of bodies suggests savage resistance to the Romans, either on the battlefield or through deadly ritual.
"Maybe the whole thing is a gigantic sacrifice ... an appeasement to the gods in order that they will get ultimate victory against the Romans," Horton said.
The Alveston cave bones hint at something even more sinister—cannibalism.
A human thighbone in the cave had been broken open in exactly the same method people use to get at the nutritious bone marrow of nonhuman animals.
But if the bone is proof of Celtic cannibalism, the practice was probably extremely rare, Horton said. It may be evidence of increasing hunger and desperation as Roman invaders closed in, he added.
"Least Bad Evidence"
Researchers have struggled in the past to link any archaeological evidence to the Druids, let alone signs of human sacrifice or cannibalism, said archaeologist Simon James of the University of Leicester, U.K.
"There has always been a suspicion that what the Romans were saying was atrocity propaganda. But some recent finds like Lindow Man suggest that there were dark and bloody goings-on," said James, who was not involved in the new documentary.
The mistletoe pollen from Lindow Man is the "least bad archaeological evidence we've got that fits in with these stories about the Druids," he added.
"Maybe mistletoe plants had been dusted on his food ritually, a bit like spraying holy water around, or dunked in his drink," James said.
If Lindow Man and others were in fact sacrificed in a bid to stop the Romans, their lives were lost in vain.
By the early centuries of the first millennium A.D., the Celts' defeat and absorption into the Roman Empire was nearly complete across Europe.
Today, their once wide-ranging culture lives on mainly in the traditional languages of Ireland, Wales, and Brittany, France.
(Read "The Celtic Realm" from National Geographic magazine.)