Researchers working in northern Siberia have been striving to restore the grassland ecosystem that supported mammoths, woolly rhinoceroses and other giants during the Pleistocene epoch, between 1.8 million and 10,000 years ago. The project, dubbed Pleistocene Park, could help settle a longstanding debate among scientists over whether human hunting or climate change brought about the extinction of the megafauna. It could also mitigate the effects of climate warming. Project leader Sergey Zimov describes the work in the current issue of Science.