Mazurian Canal, never completed, was to connect Mazurian Lakes with the Pregola River so as to get a waterway to the Baltic Sea. The construction of the canal was begun in 1911 and first time was stopped by WW1. The second stage of the construction, vigorously carried out from 1934 was interrupted by WW2, after the war the construction was never resumed.

Shelters nr 11 and 12 have such large sizes (21×13 m) that for years have been recognized as secret factory halls for manufacturing U-Boots, which then were to be transported via the lock in Lesniewo to the Baltic Sea. That is how the legend of U-Boots in Mamerki was founded.

Walking along Mazurian Canal, construction work of beavers can be admired, as they build their dams. After 4 km we reach a huge lock in Lesniewo Górne, which is 21m high, 7.5 m wide and 46m long. The construction of the loch is almost completed, yet it was never reached by the canal.

The Canal was designed for ships up to 240 tons, the route is in artificial trenches or earthen embankments. Ten huge locks were built to overcome the level difference of 111 m between the river Lyna and Mamry lake, five of them are on Polish territory.