The pearl fisheries along the Arabian coast of the Persian Gulf are also controlled by Arab traders. From there are obtained some of the finest pearls to be found, and also many tons of mother-of-pearl shells. The yearly product of the fisheries is thought to exceed more than two millions of dollars in value. The pearls are found in a species of oyster, and to obtain them the divers must go to the bottom in from thirty to ninety feet of water. Expert divers can remain under water as long as two minutes. The oysters are taken ashore to be opened, and Turkish inspectors are on hand to levy a tax on the product. A few pearls may escape him, especially if he is temporarily blinded by the glare of several piasters; but the pearl industry is taxed for about all that it is worth. Mecca, the birthplace of the Prophet Muhammad, is the city to which every disciple of Islam is supposed to make a pilgrimage at least once in his lifetime. The chief income of the inhabitants of Mecca is obtained by renting rooms and entertaining the visiting pilgrims who flock thither. In the centre of the city is the so-called Sacred Mosque, or area, which is entirely enclosed by a covered structure of colonnades having minarets and cupolas. Within the centre of this enclosed space is a cube-shaped building called the Kaaba, which contains the famous sacred Black Stone. This stone, probably of meteoric origin, gives to the building its sanctity, and is an object of the greatest veneration to every pious Moslem, who kisses it repeatedly. There is also within the enclosure a building containing the holy well, Zemzem, the only well in Mecca. No unbeliever is permitted to enter the sacred enclosure, much less to pollute the Holy Kaaba by his presence. A few infidels disguised as pilgrims, at the risk of their lives, have visited this sacred place. The preparations for pilgrimage are unique. The pilgrims assemble near Mecca during the holy month and begin the sacred rites by bathing and assuming the sacred garb. This suit consists of two woollen wrappers, one worn around the middle of the body and the other around the shoulders. With bare head and slippers covering neither heel nor instep the pilgrim sets forth on his holy journey. While wearing this dress he is admonished to bring his thoughts into harmony with the sanctity of the territory he now traverses. He is not to shave, anoint his head, pare his nails, or bathe until the end of the pilgrimage. Among the various rites to be performed after reaching Mecca is walking seven times around the Kaaba, first slowly, then quickly. Before leaving the city the pilgrim drinks water from the holy well, Zemzem.