The most compelling thing about the genre is the element of mystery. Even for the cases that have been solved — and many haven't — there's something haunting about simply knowing the details.
The r/AskReddit thread, "What is a case that haunts you to this day?" brought out all the true crime aficionados. We've avoided the gory details in the anecdotes below, but please be aware that some of the links contain disturbing details.