The Emba River (Kazakh: Zhem) in west Kazakhstan rises in the Mugodzhar Hills and flows some 400 miles (640 km) southwest into the Caspian Sea. It flows through the north of the Ust-Urt plateau, and reaches the Caspian by a series of shallow lagoons, which were navigable in the 18th century. The lower course traverses an area of salt domes and the petroleum-rich Emba fields. It is considered by some experts as a boundary between Asia and Europe and was first proposed as such by Philip Johan von Strahlenberg.