Their investigation demonstrates that the Bronze Age 'megacities' of the Indus Valley region of Pakistan and north-west India declined during the 21st and 20th centuries BC and never recovered - because of a dramatic increase in drought conditions. 

Read more at:
http://economictimes.indiatimes.com/articleshow/31332973.cms?utm_source=contentofinterest&utm_medium=text&utm_campaign=cppst