What is the most unsettling/disturbing unsolved mystery that you are aware of?

Add a link to an article or other coverage of the mystery if you can.

Here's one: The Max Headroom signal hijacking incident

In 1987, two individuals hijacked television signals of two TV stations at night and broadcasted creepy footage of someone wearing a Max Headloom mask and costume to viewers. They were never identified.

See:
Video 1
Video 2