One Scandinavian worker’s after-hours hobby led to a surprising find—a cache of rare silver Viking coins.

According to the Copenhagen Post, a man named Robert Hemming Poulsen traveled to the small Danish island of Omø in September to lay fiber optic cables. He packed his metal detector, and used it to hunt for treasure in his off-hours.

Hauling the machine all the way to Omø paid off. Poulsen ended up uncovering numerous Harald Bluetooth coins, which date back to 10th century Denmark. The coins were minted during the reign of Harold Bluetooth (958 – 987), who might have introduced the first nationwide coinage in Denmark. They’re typically hard to find, as their silver content and weight is so low that they often elude metal detectors.

Denmark’s Museum Vestsjælland excavated the site, and the treasure is now on display for history buffs to admire.  

“A treasure like this is found once every 10 to 15 years,” Hugo Hvid Sørensen, a curator from Museum Vestsjælland, told the Copenhagen Post. “It contains many items and is extremely well kept because it has been buried in sandy earth.”

[h/t Archaeology, Copenhagen Post]