In the pre-internet Stone Age of the 20th century, knowledge-seekers had only a few options when they had a burning question that needed to be answered. They could head to their local library, ask a smarter relative, or embrace the sales pitch of Time-Life Books, the book publishing arm of Time Inc. that marketed massive, multi-volume subscription series on a variety of topics. There were books on home repair, World War II, the Old West, and others—an analog Wikipedia that charged a monthly fee to keep the information flowing.

Most of these were successful, though none seemed to capture the public’s attention quite like the 1987 debut of Mysteries of the Unknown, a series of slim volumes that promised to explore and expose sensational topics like alien encounters, crop circles, psychics, and near-death experiences.

While the books themselves were well-researched and often stopped short of confirming the existence of probing extraterrestrials, what really cemented their moment in popular culture was a series of television commercials that looked and felt like Mulder and Scully could drop in at any moment.

Airing in the late 1980s, the spots drew on cryptic teases and moody visuals to sell consumers on the idea that they, too, could come to understand some of life's great mysteries, thanks to rigorous investigation into paranormal phenomena by Time-Life’s crack team of researchers. Often, one actor would express skepticism (“Aliens? Come on!”) while another would implore them to “Read the book!” Inside the volumes were scrupulously-detailed entries about everything from the Bermuda Triangle to Egyptian gods.

Mysteries of the Unknown grew out of an earlier Time-Life series titled The Enchanted World that detailed some of the fanciful creatures of folklore: elves, fairies, and witches. Memorably pitched on TV by Vincent Price, The Enchanted World was a departure from the publisher’s more conventional volumes on faucet repair, and successful enough that the product team decided to pursue a follow-up.

At first, Mysteries of the Unknown seemed to be a non-starter. Then, according to a 2015 Atlas Obscura interview with former Time-Life product manager Tom Corry, a global meditation event dubbed the "Harmonic Convergence" took place in August 1987 in conjunction with an alleged Mayan prophecy of planetary alignment. The Convergence ignited huge interest in New Age concepts that couldn’t be easily explained by science. Calls flooded Time-Life’s phone operators, and Mysteries of the Unknown became one of the company’s biggest hits.

"The orders are at least double and the profits are twice that of the next most successful series,'' Corry told The New York Times in 1988.

Time-Life shipped 700,000 copies of the first volume in a planned 20-book series that eventually grew to 33 volumes. The ads segued from onscreen skeptics to directly challenging the viewer ("How would you explain this?") to confront alien abductions and premonitions.

Mysteries of the Unknown held on through 1991, at which point both sales and topics had been exhausted. Time-Life remained in the book business through 2003, when it was sold to Ripplewood Holdings and ZelnickMedia and began to focus exclusively on DVD and CD sales.

Thanks to cable and streaming programming, anyone interested in cryptic phenomena can now fire up Ancient Aliens. But for a generation of people who were intrigued by the late-night ads and methodically added the volumes to their bookshelves, Mysteries of the Unknown was the best way to try and explain the unexplainable.