It's home to lots of rare animals, such as wild European bison and wild boar.
Nature campaigners say it is as important as the Amazon rainforest in South America.
They are annoyed because a ban on cutting down trees in a part of the forest has been lifted after 100 years.
Beetles
The trees are being destroyed by beetles, but the nature campaigners still think they shouldn't be chopped down.
They think nature should just be allowed to take its course.
They are now appealing to people across Europe to join their campaign to protect the forest.
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