How were the hunter-gatherers of Göbekli Tepe able to build a series of stunning stone monuments six thousand years before Stonehenge?Was the so-called 'Wow!' Signal a radio transmission from deep space, or the ambient resonating frequency of a passing comet?What happened mid-Atlantic to the passengers and crew of the Mary Celeste, leaving the abandoned ship to sail on by itself?Wonderful and weird, here are twenty incredible mysteries that continue to enthral and perplex.Each unexplained mystery, whether ancient or modern, presents the reader with its own unique challenge.