Why did Vikings bury so much Arab silver?

Because silver was both their money and their savings. The Volga route poured Arab silver north, and hoards were buried to be dug up later, by owners who sometimes never came back. Outside a few trading towns, Scandinavia did not use coinage, so its economy ran on bullion: the weight and purity of precious metal. Traders carried small portable scales to weigh silver precisely, and cut and bent pieces of silver, called hacksilver, were used as currency by weight.

The coins reached the north through trade between Rus merchants and the Abbasid Caliphate, along the Volga trade route. The Rus carried furs, honey and slaves down the Volga. The sources are not comfortable reading: a Persian geographer, ibn Rustah, wrote that they sailed to ravage the Slavs and sold their captives at Khazaran and Bolghar.

Burying silver was a kind of bank. A hoard was usually buried to be recovered later, but hoarders sometimes died or could not return. That is why so much is still in the ground. Gotland alone has yielded over 1,000 kg of silver from over 700 caches, including 168,000 coins from the Arab world, North Africa and Central Asia.

The largest, the Spillings Hoard, lay under the floorboards of what was probably a storehouse. When archaeologists tried to X-ray it at a local hospital, the plates stayed blank, so much silver was in it. The flow stopped: by the 11th century the Volga route lost its importance as silver output in the Abbasid caliphate declined.