Under Seleucus Nicator, the high-ranking official Patrocles was commissioned to lead a voyage of exploration along the coastline of the Caspian Sea. He wrote a very influential periplus, sort of like a travel guide and geographic description, of his voyage. It would shape the Hellenistic and Roman understanding of the geography of the Caspian for centuries to come.

The problem was that he made most of it up.

Patrocles claimed that the Caspian Sea was actually a gulf which led into the world-encircling ocean through a mouth at its northernmost point. Later geographers up to Ptolemy would continue to refer to it in this way; you can see an example of such a depiction in this part of the Tabula Peutingeriana. This is particularly strange because Herodotos and Aristotle both described it as an enclosed lake, and Polyclitus later argued that it must be a lake because its water was sweetish and because it contained snakes (okay, that second point isn't a very good one).

Another of Patrocles' claims was that the Oxus (Amu Darya) and Iaxartes (Syr Darya) flowed into the Caspian. Then, as now, they both flowed into the Aral Sea, which the Greeks were probably not aware of. He said that the Oxus was navigable from the Hindu Kush, so that an east-west Caspian trade route from India to the Black Sea would be possible. No such route existed and so no-one ever took it.

Historically, the Greeks had believed that the Iaxartes and the Tanais (Don river) were connected, with the Iaxartes flowing through the steppe and becoming the Tanais during its journey, from where it flowed into the Black Sea. Thus it served as something of a boundary between Europe and Asia. Patrocles' geography, by making the Caspian Gulf a feature of the northern edge of the world, made this impossible. Seleucus Nicator's idea of cutting a canal from the Caspian to the Black Sea is probably a result of this understanding. In order to give the Seleucid Empire an ideological boundary, the canal, gulf, and river would together form its northern border and connect east with west. Building a canal that would allow sailing from Central Asia to the Hellespont would allow Seleucus to perhaps outshine Darius I and his canal from the Mediterranean to the Red Sea.

According to Pliny, Seleucus and Antiochus sailed from India to the Caspian Sea with Patrocles, which is impossible unless their ships could fly and were also time machines. This might be based on a misreading of Patrocles' account. Europeans cited this voyage as proof of a northeast passage to China in the 16th century!

Nevertheless, there was a strong political element to this invented geography. The region between Indian Ocean and the Caspian Sea could form an easily identifiable landmass which the Seleucids could name after themselves, giving their empire a natural unity and a navigable edge that linked their territorial ambitions in the west with those in the east.

Alexander had wanted to launch an exploration of the Caspian as well and appointed an admiral named Heraclides to do it, but during the construction of the ships Alexander died and the project seems to failed or died with him as no knowledge of the voyage survives. If it had taken place, it might have forestalled the long-lasting and erroneous geography that later generations were given by Patrocles.