Holy Shrine of Imam Reza (PBUH)
The holy shrine of Imam Reza, the eighth imam of shia, is considered as the second largest mosque in the world by dimension and capacity. It covers an area of about 700,000 m2 (2013). Actually, it's a complex which also contains a museum, the Razavi University of Islamic Sciences, a library, etc.
A Brief History
Imam Reza was a son of Imam Musa al-Kadhim. His father was killed by the fifth Abbasid Caliph, Harun al-Rashid. The next caliph and the son of Harun, al-Ma'mun, transferred the capital from Baghdad to Merv in Khorasan and brought Imam Reza with himself by force and unwillingly made him crown prince. But when al-Ma'mun saw that the Imam gained even more popularity, he decided to correct his mistake by poisoning him. He buried Imam Reza in Tus. Afterwards, the name Tus changed to Mashhad meaning the place of martyrdom.