Ecologist Sergey Zimov and his son Nikita have created Pleistocene Park, a 14,000-hectare reserve near the Arctic town of Chersky, Russia. Founded 19 years ago, it's a grand experiment to test whether large herbivores—elk, moose, reindeer, horses, and bison—can, by grazing, bring back a grass-dominated ecosystem called the mammoth steppe. That biome dominated the northern reaches of Eurasia and North America for 2 million years, until the end of the last glacial period some 13,000 years ago. The Zimovs hope that rewilding the arctic to bring back the mammoth steppe can prevent permafrost from thawing and releasing potentially billions of tons of greenhouse gases. But they face daunting challenges in sustaining their Arctic research outpost and realizing that vision.