A Northern California couple out walking their dog on their Gold Country property stumbled across a modern-day bonanza: $10 million in rare, mint-condition gold coins buried in the shadow of an old tree.

Nearly all of the 1,427 coins, dating from 1847 to 1894, are in uncirculated, mint condition, said David Hall, co-founder of Professional Coin Grading Service of Santa Ana, which recently authenticated them.

Although the face value of the gold pieces only adds up to about $27,000, some of them are so rare that coin experts say they could fetch nearly $1 million apiece.

"I don't like to say once-in-a-lifetime for anything, but you don't get an opportunity to handle this kind of material, a treasure like this, ever," said veteran numismatist Don Kagin, who is representing the finders. "It's like they found the pot of gold at the end of the rainbow."

Kagin, whose family has been in the rare-coin business for 81 years, would say little about the couple other than that they are husband and wife, are middle-aged and have lived for several years on the rural property where the coins were found.

They have no idea who put them there, he said.

The pair are choosing to remain anonymous, Kagin said, in part to avoid a renewed gold rush to their property by modern-day prospectors armed with metal detectors.