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The curious case of the green mummy has baffled scientists for decades.
A teenager, buried in Italy hundreds of years ago, developed a distinctive emerald sheen not usually found on human remains.
Now, the secret behind the unusual colouring has finally been revealed.
And scientists say it 'completely changes' their understanding of the role of certain materials during the preservation process.
The clue to the green colour lies in the copper box the boy was buried in, the experts say.
This would have helped preserve the body's hard and soft tissues thanks to the metal's antimicrobial properties.
But it also likely reacted with acids that leaked out of the body and eroded away the box, creating corrosion products that interacted with chemical compounds in the bone.
Over time, copper ions replaced calcium in the boy's skeleton, solidifying the bone structure while tinting the affected areas various shades of emerald.
The adolescent, who was 12–14 years old when he died, was first unearthed in the basement of an ancient villa in Bologna, in northern Italy, in 1987.
He had been buried in a copper box, and his skeleton was complete except for the feet.
The discovery of any kind of mummified remains are significant to science but this was especially extraordinary as – apart from the left leg – the mummy was almost entirely green from skin to bone.
It has carefully been stored at the University of Bologna since the initial discovery. But now, a team of experts have finally revealed the mysterious circumstances that led to its noticeable tinge.
Radiocarbon dating placed the boy's death to between 1617 and 1814, Annamaria Alabiso, conservation scientist at the University of Rome Tor Vergata, said.
In–depth chemical and physical analysis shows no clear signs of disease or trauma, so it is unclear why the teenager died.
The preserved skin was covered by a pale green coating that commonly develops on copper and bronze statues.
'This completely changes our point of view on the role of heavy metals, as their effects on preservation are more complex than we might expect,' Ms Alabiso told New Scientist.
It's thought the bottom of the copper box cracked open at some point, letting the liquid out.
This meant the body would have stayed in a cool, dry chamber with little oxygen, boosting the preservative effects.
The boy's feet might have detached and got lost when the box broke, the scientists said.
'It was a very emotional experience for me to work with these unique human remains,' Ms Alabiso added.
Writing in the Journal of Cultural Heritage the team said: 'Ultimately, the Bologna green mummy is the result of a unique combination of environmental factors: the low temperatures in the basement, the limited oxygen availability inside the cist, and the biocidal action of copper ions, which collectively contributed to its peculiar mummification process.'
Other mummified body parts have turned a different colour after burial with copper or bronze objects, such as the green, mummified hand of a newborn baby clutching a copper coin.
However, this adolescent boy is the most complete green mummy found to date.
HOW DOES COPPER CAUSE MUMMIFICATION?
The unusual remains of a baby buried 150 years ago have shed new light on the bizarre effects of copper on a decomposing body.
While the remains were almost entirely skeletonized, the body contained a single mummified hand.
Further analysis revealed the child was buried clutching a copper coin, which protected some body parts from decay.
‘Copper compounds often cause superficial green coloration on human remains,’ the authors wrote in a study about the discovery.
‘Soluble copper may also penetrate soft tissues and bones.’
Flesh and discoloration could be seen in the areas closest to the copper; the dried up, flesh-covered forearm was stained green down to the bone, as were some of the vertebrae, a hip bone, and the leg bones.
The copper levels seen in the remains were several hundred times higher than average, according to the researchers.
‘While copper is essential for the functioning of some human enzymes, it is also known to have antimicrobial character and its compounds have been used as fungicides and bactericides for many years,’ the researchers explain.
Still, the process of ‘copper-driven quasi-natural mummification’ has not been documented, and this may be the first reported case.