This show feels like an early version of what you’d find on the Investigation Discovery channel, but Robert Stack's narration is much more unsettling than anything that network could produce. He comes across as both frightening and intensely serious, portraying himself as someone who knows a great deal about the subject matter at hand. Beyond that, it made an effort to encompass a wide spectrum of unexplained phenomena, aiming to attract viewers interested in everything from actual criminal cases to theories about ancient civilizations, historical mysteries, and general conspiracy beliefs. Every installment contained elements designed to engage many different kinds of audiences, establishing it as the best series within its category.

Source on TMDB