Margaret Cox searches for butterflies, but there's no joy if she finds them. They are the by-product of what she terms a "geophysical anomaly". We know it by its common name: a mass grave.

Margaret Cox searches for butterflies, but there's no joy if she finds them. They are the by-product of what she terms a "geophysical anomaly". We know it by its common name: a mass grave.

In the butterfly kingdom, each species feeds on a specific "host" plant. There is a blue butterfly that feeds only on Artemisia vulgaris or mugwort, a wild flower that grows to 4ft, about the height of a child. The people of medieval Europe stuffed pillows with mugwort believing it would give them happy, vivid dreams. They thought it magical, a charm.

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Some years ago in the Balkans, clouds of blue butterflies gathered on dense banks of Artemisia that had flourished all of a sudden. A change in the soil's nutrient levels and a disturbance of dormant seed banks had led to this dense colonisation. It was 1999, and the plants were blossoming in Kosovo.

In the 1990s, mass graves hid the victims of Kosovo's genocide in which Albanian Kosovans where killed by Serbs. But the dead are not that easy to hide. They leach into the soil, raise the nutrient level, feed the weeds, lure the butterflies. No vivid, happy dreams. Not for anyone.

Not least for Cox. On first meeting at her cramped Bournemouth office, it is easy to imagine Cox as the archetypal lepidopterist, collecting rare species for her display cabinets. A slight woman of average height with a rolling Dorset accent and healthy country glow, it is certainly hard to picture her waist-deep in the decaying remains of thousands of dead people.

But Cox swapped her wellingtons for the steel-capped boots and Tyvek suit of the forensic anthropologist a long time ago. The gentle Dorset accent remains but it does little to soften the impact of the atrocities her work involves.

It is Cox's job to locate, excavate and exhume mass graves believed to hold the victims of genocide. Her brief is twofold: to retrieve the evidence for the courts and international tribunals, enabling them to convict those responsible and, secondly, to identify and return the exhumed remains to their families. This is what she describes as "the desired end point. That remains go back and are buried by people to whom they mean something and who can then move on with their lives."

This weekend marks the 10th anniversary of the end of the Rwandan genocide: one of the atrocities Cox has investigated. For most, the mass graves of genocide symbolise the worst aspects of humanity but for Cox they are the beginning of justice: "both a symbol of despair and of hope". It is a view shared by her colleague, forensic anthropologist Dr Mark Skinner, who says: "Unexcavated, they can function as political tools to intimidate survivors; scientifically excavated they are threats to the perpetrators."

And it is this search for a just resolution to unjust events that enables Cox to continue with such seemingly hopeless work. But before the harrowing processes of excavation and exhumation can begin, and the wheels of justice finally turn, the graves must first be found.

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It is hard to imagine that the bodies of thousands of men, women and children could be hard to find, but they are. Any murderer trying to avoid prosecution hides the evidence - the body - so too with the perpetrators of genocide.

Cox has spent much of the past 10 years working for Inforce (International Forensic Centre of Excellence), an organisation she founded. She has been to Eastern Europe, Iraq and Africa with Inforce, hunting down the past with the science of the future.

Thermal-imaging can pick up temperature differences in the ground. A decaying body heats up, so mass graves packed with hundreds of bodies give a very clear thermal image. "Geophysical prospection" or "remote sensing" can detect whether there has been any ground disturbance. Soil-resistance surveys, ground- penetrating radar, magnetrometry and cadaver dogs are all used to find the buried. And aerial and satellite imagery help detect grave sites.

But as the techniques and technology of those hunting the killers increases, so too do the mechanics of concealment.

"There was one grave that was considered to be very important for indictment purposes, says Cox talking about her work in Kosovo. "It was related to the massacre of women and * children in a specific place and we were asked to find that grave. By that stage the perpetrators knew that satellite imagery was being used, so they changed their modus operandi. Instead of digging mass graves, they moved towards burying people in individual graves in legitimate cemeteries in Kosovo. The area that we had to look at had spoil from building sites put all over it - you can't use geophysics on uneven ground. Also, it had been littered with metal waste and you can't use any test that is based on any sort of magnetic-field anomaly where you have got metal. So, with circumstances like that, the first thing you have to do is remove the spoil - and hope that it is not booby-trapped."

Booby-trapping of graves is a real risk for Cox and her team. She says that in the former Yugoslavia "there was deliberate mining of graves to deter investigators. Razor-blades were put inside body cavities to either harm or deter the pathologists from doing their work. The genocide is highly systemised and disposal and deterrent is all built into the process."

Last May, in Iraq, Cox had to display courage under the very real threat of fire. "For the first time, I was very afraid. It was all so uncertain. Apparently the fighting had just about stopped, but you have no idea what you will find."

Cox was based in a palace formerly owned by a son of Saddam Hussein. At night she would sleep in an ostentatious mausoleum with gold taps (from which water rarely ran) and chandeliers the size of dining-tables. Cox worked with the Iraqis and other forensic teams to recover the remains of those murdered under Hussein - 300,000 were killed at Hilla and Musayyib alone. "The old regime," says Cox, "was propped up with the bones of the Iraqi people buried beneath the sands."

Cox comes from a working-class background and archaeology played no part of her early life. She left school at 16 and was married by 19. With children and a newsagents shop to run, her life looked set. But, in 1972, the Tutankhamun exhibition in London captivated her with the revelatory nature of archaeology. She joined an archaeology group in Dorset and took an Open University degree. She flourished, going on to college to gain a further degree before completing a PhD in record time at University College London.

Her passion marked her out in the dusty world of archaeology and Cox was asked to contribute to a television programme on the examination of human remains. By chance, a police officer wrestling with an unsolved murder case was watching. Cox was to be the missing link.

"I got a phone call asking me to have a look at the case. The police had an idea that the victim had been burnt alive in a railway carriage and then her remains scattered over a suburban backyard. I used archaeology to piece together the events prior to death and support it with evidence that the police used to get a conviction."

The use of archaeology in a forensic capacity to achieve a just resolution appealed to Cox. She became aware that evidence incorrectly analysed or removed from a scene of crime could destroy a case. Through Inforce, she highlighted the necessity for standard protocols to under- pin scene of crime work and, in 1996, created the unique and internationally acclaimed post-graduate forensic archaeology course at Bournemouth University. The course was devised for UK domestic-crime investigation but the emphasis soon shifted from the national to international because, a thousand miles away from the gentle sands of England's south coast, horrors were beginning to emerge from the soils of the former Yugoslavia - mass graves filled with thousands of bodies.

The International Criminal Tribunal for the former Yugoslavia (ICTY) needed evidence from the graves before Milosevic and his cronies could ever be tried for crimes against humanity and genocide. It recruited Cox's first intake of students and relocated them to the former Yugoslavia. Among that intake were archaeologists Jon Sterenberg and Caroline Barker, today leaders in their field.

"The core archaeological staff working in the Balkans at that time were all UK trained. Archaeology in the UK is probably the best in the world, simply because most of our archaeology has been trashed through ploughing and towns being developed over other towns. So we are used to dealing with ephemeral, very deeply stratified archaeology."

Stratified is a word that could describe Cox. She has seen the horror that hides beneath the guise of humanity. But hers is the language of the technician, she talks with the removed voice of an academic, as if her experiences are gained from text books rather than in waterlogged graves. This is Cox's coping method.

"If you do this work you have to try and stay, not above it, but separate. You have to try and categorise the work and not get involved and above all you can have absolutely no experiential link with it - you can't have that because it will destroy you."

In her are so many stories that she never speaks about - she keeps the horror inside, she covers it with efficiency an urgency of purpose. But she tells me one that haunts her. It is from the genocide in Rwanda, at Nyamata Church, an area where gang rape was apparently most widespread and most systematically organised, to the extent that men with HIV raped last. "In a vault below the church there was a very large coffin. It must have been three metres long," she says. But in the coffin was a woman of average height. She was buried in a three-metre coffin because she had been raped with a sapling tree. "The tree had been forced right through her body and the local people buried her with the tree inside her."

"It's off the clock - to think that people could be so..." she trails off. "The frenzy and the hatred and the dehumanisation - I find it absolutely appalling and absolutely shocking."

It has been a long journey for Cox - a mummified Egyptian king led her to unearth just some of the 200 million people who it is estimated have been the victims of mass murder since 1900. That search for justice saw her win the Humanitarian of the Year Award in 2002.

Before I leave, Cox reminds me of a quote from the legendary anthropologist Margaret Mead. "Never doubt that a small group of thoughtful citizens can change the world. Indeed, it's the only thing that ever has." *