As the birthplace of Muhammad and a site of the Muhammad's first revelation of the Quran (the site in specificity being a cave 3.2 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 Kaaba, by majority description Islam's holiest site, as well as being the center of the Islamic universe. Mecca was long ruled by Muhammad's descendants, the sharifs, 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 Abraj Al Bait, also known as the Makkah Royal Clock Tower Hotel, the world's second 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 Ajyad 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,arguably the whole world over, despite the fact that non-Muslims are prohibited from entering the city.