Hajj is the annual pilgrimage that Muslims are required to make to Mecca at least once in their lifetime if they are physically and financially able to do so. It is one of the five pillars of Islam. Early mosques did not have a distinctive architectural style as Islam began in Muhammad's homeland which had no native style, so mosques borrowed from neighboring civilizations. Some important mosques include the Dome of the Rock in Jerusalem, the Blue Mosque in Istanbul with its massive central dome and windows, and the mud mosque in Djenne, Mali, which is one of the strangest Islamic structures.