The Chicago Tribune reports that the university's admissions department received a package last week addressed to "Henry Walton Jones Jr." That's the main character's name in the popular film franchise that began in 1980s.
The package contained a replica of the journal from the "Raiders of the Lost Ark" film - and no explanation.
University officials turned to Tumblr for answers.
Admissions counselor Grace Chapin says one theory is that it's an "alternate reality" game where players plant clues for others.
Officials with Lucasfilm, the studio that made the movies, say they weren't responsible for the package.
University officials say they haven't decided what to do with the journal.