What is the biggest unsolved mystery in human history? 1 the Sea People they appeared from nowhere destroyed several civilizations and disappeared without a trace we still don't know their origin. 2 in early 2000s India a creature called the Monkey Man attacked people in Delhi terrorizing them though it's unclear if it was real or just mass hysteria. 3 the disappearance of the ancient Maya civilization despite extensive research no one knows exactly why they vanished in the 9th century.