This cover from Russia full of beautiful stamps includes a strip of 2 stamps (on the top) celebrating a natural UNESCO World Heritage Site:
The Curonian Spit. The Curonian Spit is a 98-kilometre long, thin, curved sand-dune spit that separates the Curonian Lagoon from the Baltic Sea coast. Its southern portion lies within Kaliningrad Oblast, Russia, and its northern within southwestern Klaipėda County, Lithuania. It is a UNESCO World Heritage Site since 2000, and it's shared by the two countries.
