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Why were copper-masked mummies buried in shallow graves in the wastes of Siberia, just shy of the Arctic Circle? Why were they laid to rest alongside 11th-century bronze bowls from Persia, about 3,700 miles (6,000 kilometers) to the southwest? And why were their skulls smashed in?

Russian archaeologists are just beginning to unravel the mysteries surrounding the remains discovered more than 15 years ago at Zeleniy Yar. Last week, The Siberian Times published a status report on the investigation.

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Scientists suspect that the mummies were well-preserved due to cold temperatures as well as all that copper, which prevented oxidation.

The remains of a red-haired man was found at the Zeleniy Yar site, protected from chest to foot by copper plating. Kate Baklitskaya / Go East / The Siberian Times

Thirty-four graves were found in the region's sandy soil, starting in 1997, but the archaeological excavation was suspended in 2002 after the locals objected. Now the work has resumed.

"Nowhere in the world are there so many mummified remains found outside the permafrost or the marshes," Natalia Fyodorova of the Ural branch of the Russian Academy of Sciences told The Siberian Times.

Fyodorova and her colleagues believe that Zeleniy Yar was a crossroads for trade in medieval times. In addition to the Persian bowls and the copper masks, the scientists found an iron combat knife, an iron hatchet, a silver medallion, a bronze bird figurine and a bronze bear buckle. Some of the mummies were covered in reindeer, beaver, wolverine or bear fur.

This mummy was found at the Zeleniy Yar site with a copper mask covering the face. Natalia Fyodorova / The Siberian Times

All of the deceased, adults as well as children, were buried with their feet pointing toward the Gorny Poluy River. That hints at a set of burial rituals that are unknown to experts.

Fyodorova also suspects that the skull-smashing was done soon after death, "to render protection from mysterious spells believed to emanate from the deceased." Similar beliefs are thought to have been behind the "vampire graves" of Bulgaria.

Tip o' the Log to The Siberian Times and Discovery News' Rossella Lorenzi.