The definition of 'civilization' on planet Earth is a very relative term today. Looking back as far as we can go in recorded history, we will understand that before our advanced society, many others existed on Earth. Traces of these lost civilizations can be found in ancient texts, legends, and myths while others have left behind great monuments and temples, that speak out to the world 'we were once here'. But just why did great civilizations abandon their mighty cities? Where trade, agriculture, and society were very well defined? Regrettably these are just some of the questions that history cannot offer an answer to. Here we have our list of great ancient civilizations whose mysterious disappearance remains a profound mystery. The Indus Valley Civilization One of the greatest ancient civilizations to disappear is without a doubt the Indus Valley Civilization or the Harappan civilizations. In the distant past, this great civilization had a total population over 5 million people,