Great Mosque of Mecca (Masjid al Haram), with Kabah in center containing the Black Stone (God's gift to Muhammad), surrounded by pilgrims. This spot represents the center of the Islamic world. Believed to be built on the site where Hagar and Ismail found water after their abandonment in the desert by Abraham. The Great Mosque is the largest religious structure in the world, covering 88 acres and able to hold over 800,000 worshippers.