A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. The earliest known depictions of sails are from ancient Egypt around 3200 BCE, where reed boats sailed upstream against the River Nile's current. Ancient Sumerians were using square rigged sailing boats at about the same time, and it is believed they established sea trading routes as far away as the Indus valley. The proto-Austronesian words for sail, lay(r), and other rigging parts date to about 3000 BCE when this group began their Pacific expansion. Greeks and Phoenicians began trading by ship by around 1,200 BCE.