Around the middle of the 15th century, when Fra Mauro (active ca. 1430-ca. 1459/1464) was dreaming of a route around Africa to the Indian Ocean, the Mare Indicum had been criss-crossed for centuries by Arab fleets sailing from the Red Sea and the Persian Gulf, through the Strait of Malacca, to Chinese ports to engage in trade in luxury objects in porcelain.

The China Sea and the Indian Ocean, identified respectively as Oceanus Cathaicus and Mare Indicum in the Marciana map, are represented as traversed by enormous vessels Fra Mauro calls “zonchi da India”. These were the typical Chinese junks characterized by a rectangular keel, four large masts, a rudder, and cabins for merchant travelers. Fra Mauro provides for the first time quite an accurate image of these ships, giving form to the descriptions of Marco Polo (1254-ca. 1324) and Odorico da Pordenone (ca. 1280-1331), whom the Camaldolite monk cites and summarizes in one of the cartouches beside his rendering. In some cases, the largest junks are shown with a smaller vessel nearby, to indicate that they sailed in fleets.