E98 series baseball cards found in Ohio attic.

When family members cleaned out their grandfather’s attic in Ohio recently, they never would’ve dreamed the treasures it held … worth millions.

According to the Associated Press, Karl Kissner (of Defiance, OH) stumbled across a soot-covered cardboard box that had been sitting under a wooden dollhouse for years. Inside, there were rare baseball cards dating back to the early 1900s.

While the names on the cards were familiar — Hall of Famers Ty Cobb, Cy Young, and Honus Wagner — the box would sit on a dresser for two weeks, until he learned the box he discovered was what experts said was the biggest, most exciting finds in the history of sports card collecting. Experts also said a find like this is likely to never happen again.

The cards come from an extremely rare series issued around 1910. Up to now, the few known to exist were in so-so condition at best, with faded images and worn edges. But, the ones Kissner found in his grandfather’s attic were nearly pristine, and untouched for more than a century.┬áThe colors are still vibrant, the borders crisp and white.

“It’s like finding the Mona Lisa in the attic,” Kissner told the AP.

Kissner and his family say the cards belonged to their grandfather (Carl Hench), who died in the 1940s. They said he ran a meat market in Defiance, and probably got the cards as a promotional item from a candy company that distributed them with caramels. Then, stored them away and forgot about them.

When Hench and his wife died, two of his daughters lived in the house. Jean Hench kept the house until she died last October, leaving everything inside to her 20 nieces and nephews. 51-year-old Kissner is the youngest of the bunch, and was put in charge of the estate.

When he eventually went through three generations of stuff his aunt kept, he came across the box, put them in a bank vault, and eventually had them authenticated.

Heritage Auctions is selling most of the Ohio cards over the next two of three years through auctions and private sales, so that it doesn’t flood the market. In all, they estimate the collection will bring $2 million or $3 million.