The identity of the Curonian Spit is closely associated with its sand dunes. The nature reserve is the place where the most exceptional natural values of the Curonian Spit are concentrated—namely, the Great Dune Ridge, featuring the landscapes of both grey and white dunes. The Nagliai Nature Reserve stretches between Juodkrantė and Pervalka, extending over more than 9 kilometers and covering an area of about 1,700 hectares. Within the reserve, there is a 1.1-kilometer-long educational trail.

The hour-long journey along the educational trail is the only way for visitors of the national park to see the Great Dune Ridge of the Curonian Spit up close. The trail winds through the Nagliai (Agila) Dune, which began forming in the 17th century due to drifting sands. Upon reaching the 50-meter-high ridge, visitors are treated to a stunning panorama of the Baltic Sea and the Curonian Lagoon.

Right at the beginning of the trail, visitors are greeted by breathtaking expanses of grey dunes. They are called "grey" because of the mosses, lichens, and sparse vegetation that blend together into a unified gray landscape. These dunes are also known as "dead dunes," a name they earned because they have stopped shifting. Once, pushed by winds blowing from the sea, the sand masses moved like living bodies toward the lagoon. Since the 17th century, these shifting sands buried more than one fishing village. In the 19th century, after the formation of a protective coastal ridge and the stabilization of the sand with vegetation, the dunes became "dead"—they no longer moved. The roots of plants continue to hold them firmly in place to this day.