The gorge of Prosek (Demir Kapija) in the valley of the river Vardar is a point of the utmost strategic importance being a link between the cities of Skopje in the North and of Thessalonica in the South. The railway between Skopje and Thessalonica was built by the Ottoman railway company "Chemins de fer Orientaux" in 1873. After the establishment of the Salonica Front, the German Army began to build and enhance roads along the front line and in its hinterland. One of these projects was the "Mackensen Tunnel" in the gorge of Prosek (Demir Kapija). The German Emperor and King of Prussia Wilhelm II (reigned 1888-1918) ordered his troops in 1916 to build a road tunnel and a road in the valley of the river Vardar, which is attested by the German inscription on the cliff above the Western entrance of the tunnel: "Wilhelm II Deutscher Kaiser König von Preussen befahl seinen Soldaten diese Strasse zu bauen 1916". Below this inscription is another one in a bay in the cliff, which reads: "Erbaut von der Baudirektion der 11. Armee 1.5.1916-8.7.1916".
The situation on the ground, which we have established in the wake of two surveys (2016, 2025), is as follows: to the South of the river Vardar the railway with its tunnel still exists, to the South of it is a tunnel, which is still used today by local traffic and which we initially thought to have been the "Mackensen Tunnel", and to the South of this tunnel there is a bay in the cliff, which is closed today by a concrete wall and a door. A careful study of German photographs from the First World War has revealed that this closed bay was indeed the "Mackensen Tunnel". This becomes evident based on a German postcard from 1917 which shows the railway with its tunnel and the "Mackensen Tunnel". Thus, we have been able to establish the relation of these two features in the landscape. On another German postcard we can clearly discern the German inscription "Mackensen Tunnel" above today's bay, which attests the tunnel's name and does not exist any more. Another German photograph from the First World War shows a car leaving the "Mackensen Tunnel", again highlighting its name. What is more, this last photograph helps us to realise that the ground level has changed substantially over the last 100 years, when we compare it to our photograph of the bay (the former Western entrance to the "Mackensen Tunnel"). The use of this nowadays closed space is unknown. Also unknown is the date when the new road tunnel was built, but surely after the First World War.