the "kaaba", a cuboid structure located within the "masjid al-ḥarām" ("sacred mosque") in mecca, saudi arabia, according to islamic tradition was rebuilt by ibrahim (abraham) and his son ismail (ishmael), and is one of the holiest places in islam. during his life the prophet muhammad laid the black stone in one of the corners of the building. many millions of muslims visit mecca and the surrounding areas each year during a pilgrimage known as the "hajj" – the fifth and final pillar of islam – during which they circumambulate the kaaba as part of the ritual.