The California couple who found a treasure trove of gold coins from the 19th century will most likely have to hand over about half of its worth to the government.

The unidentified duo, who discovered over $10 million worth of the rare coins while walking their dog in their backyard, is slated to lose nearly half after due taxes are collected. 

CBS reported that the couple must give the tax money because of a 1969 lawsuit in which a husband and wife purchased an old piano for $15, but discovered nearly $4,500 inside the belly of the instrument. They lost the case, and had to give tax money to the IRS. 

The unnamed couple must fork over some federal income tax, which could be as high as 39.6 percent. They might also encounter another state tax - an additional 12 percent to pay up.

But lawyer Alan Solarz told CBS a potential issue in calculating just how much the duo must pay for the coins is finding out their exact worth.

"That's where the rubber will hit the road in terms of a dispute with the IRS," Solarz, who works for Bryan Cave LLP in New York, told CBS. "One appraiser could say $10 million, another could say $1 million. It's not easy to value until you sell it." 

The stash of 1,427 coins, all of which are in mint condition, have been dated between 1847 and 1894. The coins are worth about $27,000 at face value, but will most likely be labeled with a much higher price due to their scarce antique qualities.

The Sierra Nevada couple told the SF Gate that the moment they found the coins was "very surreal."

"It was very hard to believe at first," the man, who wished to remain anonymous, stated. "I thought any second, an old miner with a mule was going to appear." 

Coin dealer Don Kagin, who gave the initial appraisal of the rare goods, said he was shocked by the find.

"You hear all those Wild West stories of buried treasure, and you think they're fantasies - well, here, this one really did happen. I almost fell out of my chair," he said. "It was mind-blowing. I was literally sitting with the most amazing buried treasure I've ever heard of."