The Polish government has built a steel border wall 186km long and 5m high along its eastern frontier. It is meant to stop global migrants from Asia and Africa trying to cross from the Belarusian side. But the wall cuts straight through the Białowieza forest - the largest remaining stretch of primeval forest in Europe and a Unesco world heritage site. Grzegorz Sokol meets environmental scientists, activists and local villagers, such as Kasia Mazurkiewicz-Bylok who treks into the forest with a rucksack of supplies to try to help migrants lost in the dense, trackless forest. And, Kat Nowak, a biologist trying to log the precise effects of the wall, from the plant species brought in with the gravel for the foundation, to the possible effects on wolf behaviour.