LOS ANGELES - A trove of rare Gold Rush-era coins unearthed in California last year by a couple as they walked their dog may be the greatest buried treasure ever found in the United States, worth more than $10 million, a currency firm representing the pair said on Tuesday.The 1,400 gold pieces, dating to the mid- to late 1800s and still in nearly mint condition, were discovered buried in eight decaying metal cans on the couple's land last April, said coin expert David McCarthy of currency firm Kagin's."We've seen shipwrecks in the past where thousands of gold coins were found in very high grade, but a buried treasure of this sort is unheard of," McCarthy said. "I've never seen this face value in North America and you never see coins in the condition we have here."Kagin's has declined to identify the couple, who according to the firm want to remain anonymous for fear treasure hunters will descend on their property in Northern California's so-called Gold Country, named after the state's 1849 Gold Rush.The couple had been walking their dog when they came across a rusty metal can sticking out of the ground and dug it out. After finding gold coins inside they searched further and found the rest of the cache.Also unclear is who hid the gold pieces, which were minted between 1847 and 1894, in a variety of 19th-century metal cans on land that eventually became part of the couple's yard.