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.
Nurse from Canada who reported to police that she was being stalked and harassed by someone she didn't know. She made a bunch of reports over 7 years but police never found any evidence of stalking. She then disappeared in 1989 and was found dead two weeks later tied up and choked. Cause of death was found to be a drug overdose.
Police suspected she staged the disappearance and being tied up/choked and actually committed suicide, since she had a history of depression and prior police investigations into a stalker always turned up dry.
There is some evidence suggesting she was murdered like the fact the blood in her body settled in the opposite side of the one she was found laying on, indicating she was relocated after she died.
We still don't know the exact circumstances leading up to her death.
According to investigators she injected herself with a massive dose of a fast acting drug then threw the injector away, did something with her shoes (no explanation as to what, she was barefoot and her feet were clean so must have been wearing some if she walked there herself and her shoes were not found at the scene), and then tied herself up in a painful and awkward position in the seconds it would take for the drug to affect her movements and render her unconscious.
Those investigators were working hard to deny that she had been being stalked for years while they had blamed and accused her. They didn't even notice when they passed the point of incredulity.