Crossing Continents reports from Poland’s eastern frontier, where the Polish government has built a steel border wall - 186 kilometres long and five metres high, it’s 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, which is also a UNESCO world heritage site.

Grzegorz Sokol meets environmental scientists, activists and local villagers each with their point of view. Women like Kasia Mazurkiewicz-Bylok who treks into the forest with a rucksack of supplies to try to help migrants lost in the dense, trackless forest. Or 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.

The deep and dark forest of Białowieza seems to have lain undamaged by humans since it began to grow more than 12,000 years ago. But this remote part of Poland is in reality no stranger to upheaval. Caught in the fault lines of wars and revolution throughout the 20th century, the forest's villages have been razed more than once. Villagers have been murdered, forced to flee and become refugees themselves. As Grzegorz explores the forest, these hidden histories feel ever more present.

Producer Monica Whitlock