There are some unsolved mysteries and unusual happenings in this case.
A self-confessed optimist, he says that having unsolved problems is what makes life interesting.
Only a few nomadic tribesmen with their herds of reindeer were witnesses to a mystery that goes unsolved to this day.
It is the third arrest as part of a police operation using new DNA technology to reinvestigate unsolved sexual assaults.
With a population of just over 100,000, it has had 20 unsolved murders in the past 30 years.
The unsolved murder became a cause célèbre in Northern Ireland.
Over the same period, Strathclyde had 47 detected crimes and 38 unsolved crimes.
It is an unsolved riddle which has inspired explorers and writers for nearly 80 years.
With medicine you always start with an unsolved medical problem.
Discomfort is the major unsolved problem with protective clothing.