Among the notable features of Baghdad during this period were its exceptional libraries. Many of the Abbasid caliphs were patrons of learning and enjoyed collecting both ancient and contemporary literature. Although some of the princes of the previous Umayyad dynasty had begun to gather and translate Greek scientific literature, the Abbasids were the first to foster Greek learning on a large scale. Many of these libraries were private collections intended only for the use of the owners and their immediate friends, but the libraries of the caliphs and other officials soon took on a public or a semi-public character. Four great libraries were established in Baghdad during this period. The earliest was that of the famous Al-Ma'mun, who was caliph from 813 to 833. Another was established by Sabur ibn Ardashir in 991 or 993 for the literary men and scholars who frequented his academy. Unfortunately, this second library was plundered and burned by the Seljuks only seventy years after it was established. This was a good example of the sort of library built up out of the needs and interests of a literary society. The last two were examples of ''madrasa'' or theological college libraries. The Nezamiyeh was founded by the Persian Nizam al-Mulk, who was vizier of two early Seljuk sultans. It continued to operate even after the coming of the Mongols in 1258. The Mustansiriyah ''madrasa'', which owned an exceedingly rich library, was founded by Al-Mustansir, the second last Abbasid caliph, who died in 1242. This would prove to be the last great library built by the caliphs of Baghdad.