At one point, there were 50 million wild bison in North America. But by the late 1800s, after years of unchecked commercial hunting, the animals were nearly extinct.

Then, at the turn of last century, there was a wildlife conservation movement. The government and private groups established protected lands, and ranchers set up breeding programs. Since then, the animals have made a big comeback — and gotten some significant recognition. In 2016, for example, bison became the country's first national mammal.

Journalist Amy Martin spent more than a year reporting on bison restoration efforts in Montana. What she found became part of her podcast called Threshold, "stories of life on earth."