Wikipedia has a great index page of the kind of unsolved mystery that "In Search Of" used to investigate (what a great show!).
Bigfoot, Nessie, the Bermuda Triangle, the Mary Celeste, the Flying Dutchman, the Roanoake Colony -- I loved these mysteries as a kid. Scholastic Books sold little paperbacks about them, and we kids snatched them up.
They now seem so tame and innocent compared to the slashings and slayings that are our "mysteries" today.