Expression of the seven seas
Seven Seas (Eng. Seven Seas) – idiomatic expression, since ancient times means the oceans as a whole.
The expression “Seven Seas” (as, for example, in the phrase “sailing for Seven Seas”) can refer either to a specific set of reservoirs, or to the entire World Ocean.
At different times, the expression “Seven Seas” had different meanings. Since the XIX century, the term has been used to refer to seven oceanic reservoirs:
North Atlantic Ocean;
South Atlantic Ocean;
Southern (or Antarctic) Ocean.
For the first time, the term “Seven Seas” appears 2300 years before our era in the 8th hymn of the Sumerian priestess Enheduanna, dedicated to the goddess Inanna. In Mesopotamia, for the first time in the history of astronomy, an account was taken of the observed seven moving objects in the heavens – seven classical planets of the Seven Heavens, the Sumerians also extended this sevenfoldness to the seas.
In ancient Rome, the term Seven Seas, “septem maria” (Latin) and “Ἑπτὰ πελάγη” (ancient Greek), often had a different meaning than today. The shipping network, which included numerous lakes, lagoons and isthmuses, in the mouths of the Po river, where it flows into the Adriatic Sea, was commonly called the “Seven Seas”. Pliny the Elder, a Roman writer and naval commander, wrote about these lagoons, separated from the open sea by sandbanks:
All these estuaries enter the Flavian Canal; originally dug by the Etruscans on the basis of the [Sagis mouth] alone. Thanks to this, they were able to direct the flow of [river] water through the Atrian marshes, which are now called the “Seven Seas”, [and then called Atrian] through the glorious Etruscan port city of Atria. From it is the old name of the present Adriatic Sea: Atriatic.
Original text (lat.)
… omnia ea fossa Flavia, quam primi a Sagi fecere Tusci egesto amnis impetu per transversum in Atrianorum paludes quae Septem Maria appellantur, nobili portu oppidi Tuscorum Atriae, a quo Atriaticum mare ante appellabatur quod nunc Hadriatic.
Arabs and their close neighbors called the seven seas (Arabic: بحار العالم, سبعة البحار) reservoirs through which they sailed to the East. Since ancient times, they have been areas of trade, and since the time of the Prophet Muhammad, they became places of widespread Islam.
In the 9th century AD, the Arab author al-Yakubi wrote:
China is a huge country, which can be reached by crossing seven seas, each of which has its own unique color, wind, fish and breeze, not available in others.
Original text (ar.)
From the text it becomes clear that anyone who wants to get from the Arabian coasts to China by water must cross seven seas: the Sea of Fars (بحر عرب, Persian Gulf), Larvi or Zanj (بحر لاروي, Arabian Sea ), Harkand (بحر هركند, Bay of Bengal ), Kalah Bar (بحر كلاهبار, Strait of Malacca ), Salahat or Salahit (بحر سلاهط, Strait of Singapore ), Kundrange or Cardange (بحر كردنج, Gulf of Thailand,  or Sanji (بحر صنجي, East China Sea ). Each of them is distinguished by its special color, has its own wind directions and a peculiar fauna.
There was also the concept of “seven Arab seas” (Arabic. بحار العالم, سبعة البحار), located in close proximity to the homeland of the Arabs, and in which there was constant navigation: