The bonanza emerged last year as the man and woman were walking their dog on their property in the Gold Country and noticed the top of a decaying canister poking out of the ground. Inside was what looked like a batch of discs covered in dirt from holes rotted through the can. Staying secretThe couple are keeping their identities and location secret for many reasons, the main one being to prevent treasure hunters from ripping up their land with backhoes. [...] they've allowed coin dealer Don Kagin of Tiburon, who helped evaluate some of the biggest sunken treasure finds in history, to offer the collection up for sale. "You hear all those Wild West stories of buried treasure, and you think they're fantasies - well here, this one really did happen," Kagin said the other day as he and his senior numismatist, David McCarthy, laid out dozens of the coins and cans for inspection at their office. According to "American Coin Treasures and Hoards," the bible of buried treasure finds, the biggest hoard of gold coins dug up before Saddle Ridge was a collection found by construction workers in Jackson, Tenn., in 1985. The oddest of the bunch are an 1866 $20 coin minted in San Francisco without the words "In God We Trust" on the back - the words were added to those coins, called "Double Eagles," later that year - and an 1849 $5 coin struck in the short-lived Dahlonega, Ga., mint. Professional Coin Grading Service of Irvine, one of the world's foremost coin-assessment firms, evaluated the hoard and certified that 13 of the coins are either the finest-preserved known examples of their kind, or tied for that rating. The nearest we can guess is that whoever left the coins might have been involved in the mining industry. The couple, who are in their 40s and are self-employed, told Kagin and McCarthy they want to donate some of the proceeds to the homeless and hungry in their area. Kevin Fagan is a San Francisco Chronicle staff writer.