The Curonian (Courish) Spit (Kuršių nerija; Ку́ршская коса́; Kurische Nehrung; Kuršu kāpas) is a long, thin, curved sand-dune spit that separates the Curonian Lagoon from the Baltic Sea. Its southern portion lies within Kaliningrad Oblast, Russia, and its northern within southwestern Klaipėda County, Lithuania. It is a UNESCO World Heritage Site shared by Lithuania and Russia.
The Curonian Spit stretches from the Sambia Peninsula on the south to its northern tip next to a narrow strait, across which is the port city of Klaipėda on the mainland of Lithuania. The northern long stretch of the Curonian Spit peninsula belongs to Klaipėda County, Lithuania, while the rest is part of the Kaliningrad Oblast, Russia. The width of the spit varies from a minimum of in Russia (near the village of Lesnoy) to a maximum of in Lithuania (just north of Nida).
The Curonian Spit was formed about 3rd millennium BC. A glacial moraine served as its foundation; winds and sea currents later contributed enough sand to raise and keep the formation above sea level.
The existence of this narrow shoal is inherently threatened by the natural processes that govern shoreline features. It depends on a dynamic balance between sand transport and deposition. It is geologically speaking ephemeral coast element. The most likely development is that the shallow bay inside the Curonian Spit will eventually fill up with sediment, thus creating new land.
According to folk etymology for the name of Neringa Municipality, there was a giantess girl named Neringa, who formed the Curonian Spit and helped fishermen.
From c. 800 to 1016, the Spit was the location of Kaup, a major pagan trading centre which has not been excavated yet. The Teutonic Knights occupied the area in the 13th century, building their castles at Memel (1252), Neuhausen (1283), and at Rossitten (1372). After the Thirteen Years' War (1454–1466), the spit became part of a Polish fief held by the Teutonic Knights. The spit may have been the home of the last living speaker of now-extinct Old Prussian, one of the Baltic languages.