Wola National Park near Szczeczin has a buffalo sanctuary for European bison. They're big, but smaller than American bison. And they tend to be pretty placid. Poland has approximately 15k bison out in the wild.
This is one of the now-uncovered entrances to Project Riese. It was hidden for decades before coming to light. Helped that it's in a forested rural area of Poland. And the tunnels were big enough to let cars drive - because the network was going to be huge.
Wola National Park is on an island near Szczeczin (Stettin) on the Polish-German border. They have a bison sanctuary there that you can visit. Euro bison are definitely smaller than the American version.
Outside of the guard rooms, nothing was truly finished in the excavations. They go on for miles, and some connect with each other. But they're not finished off. It was hard and brutal work with very little in true mining equipment. The Poles have done a good job showing what was what, though, with researchers having been filmed and put into holograms to explain things - in English and Polish.