This is the big one. Every year millions of Muslims travel to Saudi Arabia in the worlds largest gathering of humanity. The haj, Islams sacred pilgrimage, defines the city of Mecca and the Al Haram Mosque is like no other on earth. The name simply means The Great Mosque, and its almost unimaginably large the complex accommodates 800,000 worshippers on a regular basis and up to four million during haj. Centrally located is the Kaaba, the sacred building towards which all Muslims pray and around which devotees process hypnotically at the peak of the pilgrimage.