With a population of more than 300,000, Mazar-e Sharif is the 4th largest city in Afghanistan. It was founded in the 12th century after a local mullah dreamt of the secret site where Ali bin Talib, the Prophet's cousin and the fourth caliph of Islam, had been buried outside of Afghanistan most Muslims believe that Ali is buried in Najaf, Iraq. A shrine, which was later rebuilt as the Blue Mosque, was constructed on the site and the town of Mazar grew around it. The town, however, continued to live in the shadow of its larger neighbor, Balkh, until that city was abandoned due to disease in the mid 19th century. As a result, Mazar assumed the role of capital of the region - a position it has maintained ever since.