A male bison can weigh up to 900 kilograms - about the same as a small car. They eat grass, leaves and tree bark, and they walk in herds of around 20 to 30, led by an older female. Calves are born in spring and look a little bit like fluffy ginger cows.
By the early 1900s, European bison had been hunted so much they had almost vanished. Only a few survived in zoos and parks. Scientists in several countries started a huge project to save them - breeding them carefully, then releasing them back into wild forests.
Romania began bringing bison back to the Carpathians in 2014. Small groups were carefully released into the forest, where there hadn't been wild bison for 200 years. The first calves born free in Romania for two centuries arrived a few years later. The herd has been growing steadily ever since.
Bringing a species back is called 'rewilding'. It is slow work - the bison need quiet, lots of forest, and rangers who keep an eye on them. But when it works, the whole forest changes. Their hooves open up new ground for plants to grow. Other animals follow where they go. It is one of the most hopeful conservation stories in Europe today.