They report that between the Indian Sea and the Chinese Sea rises a chain of mountains that separates the two, pierced by valleys through which ships pass. The One who created them smoothed those passages, just as He has done on land as well. The Almighty says:
“We placed firm mountains upon the earth lest it should sway with them, and We opened within it broad passes so that they might find their way.” (Q 21:31)
Ptolemy, in his book known as the Almagest—which was rendered into Arabic in the time of the caliph al-Maʾmūn and forms the foundation of these sciences—states that the seas branching out from the western, eastern, southern, and northern encircling ocean are exceedingly numerous. Some are in fact one continuous body of water but are labeled according to the lands that border them. Among them are: the Sea of Qulzum, named after a village on its shore near Ayla; the Persian Sea; the Khazar Sea; the Sea of Warank; the Sea of the Romans; the Pontus Sea; and the Blue Sea, upon whose shore stands a city of that name—also called the Sea of Qirim. This latter narrows until it pours into the Roman Sea south of Constantinople, forming what is known as the Constantinopolitan Gulf. Because of this, ships travel swiftly from Qirim to the Roman Sea, yet move slowly when sailing from Alexandria back to Qirim, forced to contend with the opposing current.
This is one of the marvels of the world: every flowing body of water is fresh—except this one—and every stagnant sea is salty and brackish, save what is reported of the Khazar Sea (also called the Seas of Gurgan and Ṭabaristān), in which travelers say there is a large stretch of pure, sweet water.
Astronomers describe that sea as roughly circular, though somewhat elongated; others liken it to a triangular sail. It is not connected to the encircling ocean but stands by itself, its length estimated at eight hundred miles and its breadth at six hundred—some say even more, and Allah knows best.
Another of these seas produces the ebb and flow observed near Basra; a similar phenomenon occurs in the lands of the Maghrib. The water rises from the first night of the lunar month and continues to grow until the fourteenth night—this is the flood tide—then begins to recede, the ebb, until the month’s end. Scholars have detailed the limits of these seas, their courses, and their termini, and they have described what lies within