A single brown fingernail lies on the leather bag of his chest, which tapers to nothing where the peat-cutting machine chopped him in two. His arm lies next to him, but these fragments of a body would mean nothing, were it not for the look on his face. A face that is 2,000 years old is not expected to have a "look". Death destroys individuality - but not his. When the remains came rising out of a Cheshire bog in 1984, that deflated torso would turn out to be packed with biological information, clues to a violent death, but it's all there for anyone to see, the full horror of it, in his face. It is the face of the eternal victim, bound and garrotted and thrown into the marsh.  Lindow Man - because the marsh that maintained him in dark seclusion for so long is Lindow Moss - goes back on view today in a state-of-the-art, refrigerated and transparent grave in the British Museum's excellent new prehistoric galleries. This sensational exhibit is gently shielded in a dark bower of chippings. You have to look for him, but discretion won't stop the crowds gathering to stare at those bristling facial hairs, that perfect ear.  When we look at Lindow Man, we are looking at the present, not the past. This man's death is happening now, before your eyes, you can see him suffer. His face has the power of some howling horror mask painted by Rubens, but pain and time, not art, made this. The way the skin has aged in its peat prison enhances, rather than confuses, the man's contorted features. His eyes are clenched and lowered in infinite sadness, his neck bent so his head rests hopelessly on his chest, as if in final surrender. He gives up the ghost as they take him out to kill him. And killed he was: around his throat is the tight cord that was used to garrotte him, left tied around his torn neck when they threw him in the bog. When the bog man was forensically examined, this turned out to be just one of the brutalities perpetrated on him. First, his head was savagely beaten, then he was garrotted, then his throat was cut. In his stomach was a last meal of wheaten griddle cakes and mistletoe. That last item betrays his killers - and they're still around.  Today is the summer solstice, and the druids have taken over Stonehenge to commemorate their ancient rites. Today's festival at Britain's most charismatic monument is based on a cultural fantasy, behind which are dark facts. Before 55BC, when Julius Caesar led the first Roman expedition to these islands, there is absolutely no written record of anything ever happening here - not a name, not a date, not a famous battle. Yet colossal remains from the neolithic and Bronze Age periods dot the islands, from Stonehenge, Avebury and Silbury Hill to the astral chamber of Maes Howe on Orkney. No wonder people have made up stories to explain this world without written records or realistic art to humanise it. Only recently, archaeologists held that Stonehenge was created by visitors from the early Greek civilisation of Mycenae. Since 1995, new dating methods have put its construction back into the neolithic period - that is the late stone age: it was gradually created over a long period from about 3000BC to about 1700BC.  There is no denying the beauty of prehistoric artefacts. You can disguise it, though, by displaying hundreds of arrowheads in cases that don't seem to have been dusted since the 1950s - which is why this new set of modern and gracious galleries comes as a joy. The real test of the dynamic new British Museum is how well it does its basic job of displaying the ancient past. Very well, is the answer from looking at this new suite. It's simple and smart, so as you walk along, you perceive changing time as a succession of colours: the neolithic becomes a room full of widely spaced white objects, the Bronze Age oxidised green, the Iron Age warm, earthy brown.  The fascination of the objects is how little anyone knows about them. The Folkton drums - three solid chalk cylinders carved with abstract patterns, two of them with what look like eyes - were found in the 19th century in a barrow grave in Yorkshire. They are now dated to the third millennium BC - the time of Stonehenge - yet nothing is known of their function or meaning. Maybe you agree with me that they look like faces. What's puzzling is how they have stayed in my mind with a ghostly vividness. So have a set of carved stone balls from neolithic Scotland with their geometrical patterns, and a stone slab from Dorset with daggers and axe heads carved into it. This connects the displays directly with Stonehenge, where dagger carvings have also been found, and to the druids.  Looking at the prehistoric objects in the British Museum is mind-boggling, but reading about them is frustrating. It's almost better to walk away with an eerie memory than to read archaeological accounts that cannot get beyond physical descriptions of what is there, except through speculation. An influential idea in current archaeology is that stone was associated by these early people with the dead, while wood was closer to the world of the living. Art history might make proponents of this idea think twice. Stone doesn't always signify death - Bernini's fountains are hardly deathly. Does modern archae- ology have any more purchase on Britain's prehistoric past than the loony ideas it rejects? The medieval chronicler Geoffrey of Monmouth claims Merlin flew Stonehenge across the sea from Ireland. But it was the 18th- century archaeologist William Stukeley who launched the most potent myth of British history when he published his book Stonehenge: A Temple Restored To the British Druids.  Like today's archaeologists who argue that Britain had its own sophisticated culture before the Roman invasion, Stukeley was an anti-imperialist. In arguing that all Britain's megalithic art was created by the druids, he turned Roman propaganda on its head, since they claimed their conquest of Britain was justified to suppress this supposedly savage religious order. "An incalculable debt is owed to the Romans who destroyed their monstrous practices," wrote Pliny the Elder. The first Roman observer of the druids, Julius Caesar, was not only a great general but a pungent writer: his own account of his campaigns in Gaul and Britain is still sensational reading not least because of what he says about the druids. He accuses them of human sacrifice: sometimes they even put their victims inside a gigantic wicker man, he says, and burned them alive.  Stukeley turned the savage druids described by Caesar, Pliny and other Roman writers into philosopher-bards; indeed, Caesar admits the druids held long discussions about the nature of the universe. For an 18th-century radical like William Blake, the druids and Stonehenge represent a lost British utopia, and the tradition lives at the solstice.  The truth is the druids had nothing to do with Stonehenge. They flourished more than 1,000 years after it was abandoned. It was in the Iron Age, on the eve of the Roman conquest, that they congregated in oak groves and cut their sacred mistletoe.  What's that? Mistletoe? "The Druids hold nothing more sacred than mistletoe," says Pliny the Elder. So here, it would seem, are the killers of Lindow Man, who was given mistletoe before he was sacrificed to the gods.  The Foxton drums could have been created by aliens; walk back to the bog man and you're in our world. In the Iron Age section of the new galleries prehistoric art gets more "human". There are three little bronze heads of moustached men. The sacrificial victim hurled in a bog has something in common with masterpieces of Celtic art thrown ritually into rivers - like the museum's horned helmet found in the Thames.  Lindow Man may have been a willing sacrifice, and there's a weakness, an acceptance in his foetal form. The druids didn't create Stonehenge, but they probably created this figure of pity, with the help of peat and centuries. This treasure from our prehistory confronts us with the seduction of violence and death, the monstrosity we're inches from, the belief that a person might make a good sacrifice. Imagine what it was like for a civilised Roman in this land, says Joseph Conrad's Marlow in Heart of Darkness: "He has to live in the midst of the incomprehensible ... And it has a fascination, too, that goes to work upon him. The fascination of the abomination."  Prehistoric and Roman Britain galleries, opening today at the British Museum, London. Details: 020-7636 1555.
A murder mystery preserved in peat is at the heart of the British Museum's revamped prehistoric galleries. Jonathan Jones turns detective.