Before World War II saw the historical region of Masuria in East Prussia fall into Polish hands, German engineers had long dreamt of building a canal to connect the Great Masurian Lakes to the Baltic Sea. Construction of the Masurian Canal eventually began in 1911 but was interrupted by war and political turmoil in Germany—until the war put an end to the project, seemingly for good. Today, the canal’s partially completed locks are scattered across northern Poland and Russia’s Kaliningrad oblast, with the most impressive ruins located near the Polish village of Leśniewo.

The Nazis recognized the strategic value of East Prussia for their war operations in Eastern Europe—after all, Hitler chose to locate his wartime command post there—and began more intensively developing the canal during the 1930s. It has been speculated, although never proven, that the Nazis intended to use the canal for U-boat transportation. It appears too much of a coincidence, for example, that the southern terminus of the Masurian Canal, a small village called Mauerwald (now ‘Mamerki’), is where the Nazis chose to build a bunker complex for the Supreme Command of the Wehrmacht (Oberkommando des Heeres), known as the OKH Mauerwald.

Either way, the Nazis quite literally left their mark on the project: the upper lock at Leśniewo displays a prominent impression in the concrete where the eagle and swastika used to sit.

Leśniewo is home to two abandoned locks of the Masurian Canal. The upper lock, the larger and more impressive of the two, has now been supplemented by a small rope park, providing visitors the opportunity to safely climb to the top of the massive concrete structure. For those who feel brave enough, the lock’s cavernous interior has been outfitted with a zipline and pendulum swing. However, the area is freely accessible, and visitors can visit the lock exteriors without paying for entry via the rope park.