Sometimes, the universe decides to troll the whole human race and create something that simply cannot be explained.
As if to remind us we're not really in charge, that all our precious science and logic aren't enough to explain it all, homo sapiens is confronted with something so baffling the only recourse is to shrug and say 'it's a mystery'.
Some such mysteries are perfect storms of circumstances just right to create an end result that both fascinates and bamboozles. Others are deliberate obfuscations by someone trying to cover their tracks and hide the truth.
More than one could conceivably be deliberate fakes created solely to cause everyone to scratch their heads and make up stuff about aliens and time travel.
However they came about, such unsolved conundrums seem tailor-made to twist mankind's collective melon and force anyone desperate for answers to concoct ever more bizarre theories.
What is the cause of these historical puzzles? Ancient aliens? Psychic powers? Ruptures in the time-space continuum? Magic? Almost certainly not, but when confronted with some of these intractable mysteries, the human mind goes to some pretty weird places.