Kuršių nerija (Curonian Spit) is a 98 km long narrow spit of sand dunes between Baltic Sea and Curonian lagoon. It's possible the highest coastal sand dunes in Europe, some up to 67 meters high. More information about geology of the area is available in Kursiu Nerija National Park website.
The whole Curonian Spit is listed as UNESCO World Heritage Site since 2000.