The exact cause of death and burial site of Genghis Khan is unknown, because all of the 2000 funeral guests and 800 soldiers were killed in order to preserve the secret.
16
Since 1982, a radio frequency emanating from Russia has been broadcasting a buzzer-like sound and, periodically, an unknown voice reads orders, names, and numbers. No one knows where exactly it is coming from or what it means.
13
In 1518, a "dancing plague" struck Strasbourg, Alsace, whereby hundreds of people danced fervently in the streets over the period of a month. Some suffered heart attacks or strokes, and many others died from sheer exhaustion. It remains unexplained.
19
The Platypus used to be considered a myth like Bigfoot and the Yeti.
18
There is a mysterious and loud radio signal known as 'the space roar' that has yet to be explained.
15