Many fire temples, with their four axial arch openings, were usually turned into mosques simply by setting a "mihrab" (prayer niche) on the place of the arch nearest to "qibla" (the direction of Mecca). Zoroastrian temples converted into mosques in such a manner could be found in Bukhara, as well as in and near Istakhr and other Persian cities. Urban areas where Arab governors made their quarters were most vulnerable to such religious persecution, great fire temples were turned into mosques, and the citizens were forced to conform or flee. Many libraries were burnt and much cultural heritage was lost.