As the birthplace of Muhammad and the site of Muhammad's first revelation of the (specifically, a cave 3 km (2 mi) from Mecca), Mecca is regarded as the holiest city in the religion of Islam and a pilgrimage to it known as the Hajj is obligatory for all able Muslims. Mecca is home to the , by majority description Islam's holiest site, as well as being the direction of Muslim prayer. Mecca was long ruled by Muhammad's descendants, the , acting either as independent rulers or as vassals to larger polities. It was absorbed into Saudi Arabia in 1925. In its modern period, Mecca has seen tremendous expansion in size and infrastructure, home to structures such as the Al Bait, also known as the Royal Clock Tower Hotel, the world's third tallest building and the building with the largest amount of floor area. Due to this expansion, Mecca has lost some historical structures and archaeological sites, such as the Fortress Today, more than 15 million Muslims visit Mecca annually, including several million during the few days of the Hajj As a result, Mecca has become one of the most cosmopolitan and diverse cities in the Muslim world,despite the fact that non-Muslims are prohibited from entering the city
