Anti-tank obstacles were part of Molotov Line fortifications, there are stil few remained anti-tank ditches. However, even in places where they don't exist anymore, you can trace them - some are still marked on old topographic maps, ditches leveled and plowed are sometimes visible at satellite images as a slightly different color belt running across the field. Some ditches are now... cemeteries, because the Nazis buried there killed people.
There is one part of well preserved anti-tank trench in the Center of Resistance Prosienica (part of Zambrów Fortified Region). Here you can find soviet secret weapon - combat woodland strawberries! Woodland strawberries cause significant slowing of movement of enemy troops which are stopping, and beginning to eat them. Watch out, it is a trap for geocachers, instead of looking for next geocaches, you may get stucked in a trench full of woodland strawberries.
Geocache? Is buried under the moss-covered, sloping stump on the wall of the ditch. Shovel is useful, especially to hide again geocache in the sand.
You can find many geocaches hided here in project Molotv Line. See maps at opencaching.pl: CoR Prosienica, CoR Podbiele, CoR Dąbrowa (all in Zambrów Fortified Region).