The wall was eighteen feet tall, made of steel, topped with concertina wire, and equipped with cameras and sensors. Built by Poland in 2022, it cuts through the heart of the Białowieża Forest, a UNESCO World Heritage site that is Europe’s last great lowland primeval forest, largely untouched by humans. The forest, which straddles Poland and Belarus, is famed for its old-growth trees and its rich biodiversity: it is home to thousands of plant and animal species, from endangered fungi and lichen to lynx, wolves, and the largest free-roaming herd of European bison. In the past four and a half years, Białowieża and neighboring forests have also become a route for thousands of migrants, primarily from the Middle East and Africa, seeking to enter Europe. Ahmed (his name, like those of the other migrants in this story, has been changed for his safety) climbed the ladder and jumped into Poland. He fell on his arm and his shoulder, fracturing bones. Then it was time to run.