It was like something out of a Hollywood movie — a musty attic in a stately old home, a forlorn-looking dollhouse perched against one wall and underneath it, a mysterious dust-covered box wrapped in twine.

But what Karl Kissner and his cousin, Karla Hench, stumbled upon on that gray February day was a real-life bonanza of about 700 baseball cards from the early 1900s with a total estimated value of about $3 million, according to some experts.

"Some people discover art in their attic, others discover natural gas on their property," said Chris Ivy of Heritage Auctions in Dallas, which bills itself as the world's largest collectible auctioneer. "These people discovered cardboard gold."

Known as the "Black Swamp Find," many of the cards figure to be a prime attraction at the National Sports Collectors Convention that runs Wednesday through Sunday at the Baltimore Convention Center.

Ivy calls it "the most significant find in the history of card-collecting." And Heritage will offer 37 of the mint-condition cards of such legendary players as Honus Wagner, Ty Cobb, Cy Young, Christy Mathewson and Connie Mack at a public auction Thursday at Camden Yards.

The 1912 Boston Red Sox World Series trophy is also scheduled to be part of Heritage's auction package, and some reports say it could be sold for more than $300,000.

The story of the find of rare baseball cards begins in the small northwestern Ohio town of Defiance, in the Black Swamp region of the state.

At the end of February, Kissner and Hench were cleaning out the century-old home of their grandfather, Carl Hench, who died in the 1940s and left the house to two of his daughters. One, Jean Hench, lived in the house until last October when she died, leaving everything inside to her nieces and nephews.

"She was a bit of a pack-rat," Kissner, 52, said recently.

Which sounds like a huge understatement. It took months of family members wading through boxes and bags and trunks and drawers — the accumulation of at least two lifetimes — before Kissner and Hench finally got to the attic.

When they discovered the dirty cardboard box and opened it, they beheld a strange sight.

"The cards weren't normal," Kissner recalled. "They were real tiny. … we didn't know what we had."

"They could have tossed them in the trash right there and let millions of dollars slip through their fingers," Ivy said.

But the two cousins recognized some of the names of the players. And after a few days of researching old baseball cards on the Internet, Kissner was fairly certain these were so-called "tobacco" or "caramel" cards that came with cigarettes and candy at the turn of the 20th century.

Kissner said his grandfather, a German immigrant who ran a meat market in Defiance, probably acquired the cards shortly after they were first marketed. And at some point, the old man must have stashed them away in the attic and forgotten about them.

He sent eight of the cards overnight — insured, he hastens to add — to Heritage Auctions along with a note: "Call us before you open this."

Ivy, who heads up Heritage's sports division, and Peter Calderon, a baseball card expert, put Kissner and Hench on speakerphone as they opened the package.

"The first thing you hear is dead silence," Kissner recalled. "Then you hear 'Oh, my God!' That pretty much told us we had the real thing."

Ivy still marvels at the condition of the cards he handled that day.