From late 535 AD to 536, written records from across the world suggest a mysterious climate catastrophe. Dubbed the year "without a summer", the sun was completely dimmed and shadows were invisible even at noon. The cause of of the "worst year to be alive in history" has been long uncertain. Was it a comet? An asteroid? A volcano? Archaeologist David Keys reveals the latter is to blame for the Dark Ages of famine and plague that shaped the world order of today.