The focal point for every Muslim and the biggest mosque in the world, Al Masjid Al Haram is able to host a million worshippers and covers an area of 356,800 sq metres. At its epicentre is the Holy Kaaba, covered in black and gold cloth, around which Muslims can be found circumnavigating night and day (known as tawaf). It's the holiest structure in all of Islam, and is at the heart of the Islamic pilgrimages (hajj and umrah). The Kaaba predates the Prophet Muhammad's lifetime; Muslims believe it was built by the Prophet Ibrahim and his son Ishmael. The foundations of the mosque around the Kaaba date to at least the 7th century, when the second caliph, Omar Bin Al Khattab, built a structure to accommodate the growing number of pilgrims each year. Set into the Kaaba's eastern corner is the Black Stone, a relic Muslims believe fell from the heavens and was placed into the corner by the Prophet Ibrahim. The stone came away from the Kaaba during the Prophet's lifetime, and it is said he then personally placed it back into the corner, where it sits today. At the Kaaba's northwestern edge, a curved area known as the Hatem or Hijr Ishmael represents the area believed to have been part of the Kaaba's original boundary when it was built by the Prophet Ibrahim. The mosque is in the midst of the Third Saudi Expansion, which will increase the mosque's area to 400,000 sq metres and allow 2.5 million people to pray inside it.