Masjid al-Haram: is the largest mosque in the world. Located in the city of Mecca, it surrounds the Kaaba, the place which Muslims worldwide turn towards while offering daily prayers and is Islam's holiest place. The mosque is also known as the Grand Mosque.
The current structure covers an area of 356,800 square metres (88.2 acres) including the outdoor and indoor praying spaces and can accommodate up to four million Muslim worshippers during the Hajj period, one of the largest annual gatherings of people in the world.
Kaaba: is a cube-shaped building in Mecca, Saudi Arabia, and is the most sacred site in Islam. The Qur'an states that the Kaaba was constructed by Abraham and his son Ishmael, after Ishmael had settled in Arabia. The building has a mosque built around it, the Masjid al-Haram. All Muslims around the world face the Kaaba during prayers, no matter where they are.
One of the Five Pillars of Islam requires every Muslim to perform the Hajj pilgrimage at least once in his or her lifetime if they are able to do so. Multiple parts of the Hajj require pilgrims to walk seven times around the Kaaba in a anticlockwise direction (as viewed from above). This circumambulation, the Tawaf, is also performed by pilgrims during the Umrah (lesser pilgrimage). However, the most dramatic times are during the Hajj, when about 60 million (officially) pilgrims simultaneously gather to circle the building on the same day.