A family, cleaning out old boxes from their attic, found a box of baseball cards collected by their grandfather over 100-years ago.

Experts say the trove of about 700 nearly mint cards just might represent the greatest and rarest discovery in the sports card industry’s history. The best of the collection is expected to fetch more than $500,000 at the National Sports Collectors Convention next month in Baltimore while the entire stock could bring in $3 million.