SAUDI ARABIA // Mecca is off limits to non- Muslims. But to those who practise Islam, the desert city is the holiest of holy places. Its spiritual centre is the ornate white and green Al Masjid Al Haram - the Great Mosque of Mecca - the biggest mosque in the world. Spread across 356,800 sq metres (3.8 million sq ft), it can hold a million worshippers. It wraps around the Kaaba, the black- and gold- covered cube that is the holiest site in Islam, sometimes called the House of God. When Muslims around the world pray, they orient their prayer mats towards the Kaaba. Though the mosque is ancient, the skyscrapers throwing shadows over the Kaaba remind you that this is very much the 21st century. During the hajj, Mecca hosts one of the world's largest human gatherings. All Muslims who are physically and financially able are expected to journey to Mecca at least once in their lifetimes; each year about 2.5 million answer the call. The city becomes a teeming sea of white-clad pilgrims, praying, drinking from the Zamzam Well, and sleeping everywhere, from luxury hotels to Japanese- style nap capsules.