One of the most valuable privately-owned coin collections is headed to auction later this year after spending more than a century shrouded in secrecy.

The collection once belonged to Danish businessman and butter tycoon Lars Emil Bruun, who spent decades compiling the nearly 20,000 coins, bank notes, and medals that comprise the set. For over a century, the collection has been kept out of the public eye due to a stipulation in Bruun's will that forbade the coins from being sold until a century after his death in 1923 at 71.

Emotionally impacted by the devastation of World War I, Bruun determined the collection should be kept as backup reserves for Denmark should a second war ravage Europe.

The collection in total is estimated to be worth up to $72 million, making it the most valuable coin collection ever to go to sale.

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