Baseball card collectors and experts are stunned by the accidental discovery of a priceless set of century-old baseball cards by a family in Ohio.

The family, from the town of Defiance, is now the proud owner of a rare set of American baseball cards, after Karl Kissner stumbled upon a pristine set of cards while cleaning out his grandfather’s attic.

Kissner found the cards in a soot-covered cardboard box hidden under a wooden dollhouse.

Inside the box Kissner found hundreds of cards, tied in stacks with twine. Kissner noticed right away that the cards were smaller than the ones he was familiar with.

He also noticed some of the names on the cards: Ty Cobb, Cy Young and Honus Wagner.

Though he recognized the baseball legends, he didn’t think much more about the cards and put the box on a dresser and continued his cleaning.

It wasn’t until two weeks later that he discovered that on that day, he had actually found what some experts say is one of the most exciting finds in the history of sports card collecting.

Experts estimate the exceptional cards are worth millions -- the holy grail of baseball card collections.

The cards come from a very rare series that was issued around 1910, known as the E98 series.

Up until Kissner’s discovery, the few known cards belonging to the series were in poor condition. The cards in existence often had faded images and worn edges.

But the cards from the attic are in near perfect condition, as they lay untouched for more than 100 years. The cards Kissner found have bright, vibrant colours and crisp, white borders.

"It's like finding the Mona Lisa in the attic," Kissner said.

Experts who authenticated the cards said they may never see something as impressive ever again.

The jewels of the bunch -- 37 in total -- are estimated to garner a total of $500,000 when they are sold in August at an auction during the National Sports Collectors Convention.

There are a whopping 700 cards in total, which experts predict may fetch up to $3 million.

Besides Cobb, Young and Wagner, the collection also includes legends Christy Mathewson and Connie Mack.

Baseball fans are also in awe of the cards.

Bill Huggins, director of the House of Cards baseball memorabilia company, told CTV’s Joy Malbon that the collection is almost too good to be true.

“Nobody’s ever dumped this many of a small collection on the hobby and oh my God the condition is ridiculous!” he said.

Kissner and his family say the cards belonged to their grandfather, Carl Hench.

Hench ran a meat market in Defiance, and the family believes he got the cards as a promotional item from a candy company that gave them out with caramels.

They believe that Hench, who died in the 1940s, gave some cards away and kept some for himself.

The house remained in the family until last October, when Hench’s daughter Jean passed away, leaving everything to her nieces and nephews.

Kissner, the youngest of the bunch, was put in charge of the estate. Knowing that his aunt was a notorious pack-rat, Kissner and cousins started the enormous task of cleaning out the house.

It was months before they got to the attic and the cards.

After a few weeks of storing the box in his office, Kissner realized they may be worth something and sent eight of them to expert Peter Calderon at Heritage Auctions in Dallas.

No stranger to baseball memorabilia, Calderon had recently helped to sell the baseball that rolled through the legs of Boston Red Sox first baseman Bill Buckner from the 1986 World Series for a cool $418,000. Still, he remembers the shock he felt when he first received the cards.

"I was in complete awe," he said. "You just don't see them this nice."

The experts at Heritage Auctions did a series of checks before authenticating the cards.

The cards were then sent to Professional Sports Authenticator, which had previously authenticated less than 700 E98s.

The verdict? The Ohio cards were the best examples from the E98 series the company had ever seen.

The company scores cards on a scale of 1-to-10 based on their overall condition.

Before the Ohio cards the highest grade the company had ever given a Ty Cobb cared from the series was a seven. The 16 Cobb cards from the attic were graded a nine -- near perfection.

And the crown jewel? A Honus Wagner from the set was given a 10 -- a first ever for the E98 series.

"This is probably the most interesting find I've heard of,” said retired sports card auctioneer Barry Sloate.

Heritage Auctions says they plan to sell the Ohio cards over the next three years through auctions and private sales.

The Kissner and the Hench family plan to evenly divide the cards and the money among the 20 cousins named in their aunt’s will. Only a few said they will keep the cards rather than sell them.

"These cards need to be with those people who appreciate and enjoy them," Kissner said.

With a report from CTV’s Joy Malbon and files from The Associated Press