Known. A homeowner struck silver while clearing ground for a new patio.
The hoard is at Rebild, in Denmark. It is thought to have been buried around the 10th century. It weighs about 18.5 kilograms and holds about 700 pieces. Those pieces include silver bars, arm rings, coins, cut silver, and a small hammer of Thor. About 320 pieces, weighing 6.4 kilograms, were still inside a clay pot. Two silver pennies were minted for Edward the Elder, who ruled from AD 899 to 924. One silver bar has runes that may be a name.
The page says nobody knows why the hoard was buried.
https://www.ancient-origins.net/news-history-archaeology/viking-silver-hoard-denmark-00103141