The Kuma–Manych Depression , is a geological depression in southwestern Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

 that separates the Russian Plain (north) from the Fore-Caucasus (south). It is named after Kuma and Manych rivers.

It is sometimes regarded as a definition for the natural boundary between Europe and Asia, although most modern sources use the Greater Caucasus watershed instead.
The proposal of the Kuma–Manych Depression as a continental boundary is originally due to Philip Johan von Strahlenberg

Philip Johan von Strahlenberg

Philip Johan von Strahlenberg was a Swedish officer and geographer of German origin who made important contributions to the cartography of Russia. Strahlenberg was born in Stralsund, which then belonged to Sweden, and his original name was Philip Johan Tabbert. He joined the Swedish army in 1694...

 (1725), and was officially endorsed by Peter II of Russia

Peter II of Russia

Pyotr II Alekseyevich was Emperor of Russia from 1727 until his death. He was the only son of Tsarevich Alexei Petrovich, son of Peter I of Russia by his first wife Eudoxia Lopukhina, and Princess Charlotte, daughter of Duke Louis Rudolph of Brunswick-Lüneburg and sister-in-law of Charles VI,...

in 1730.