Spending an entire winter researching on an ice floe in the Arctic Ocean was, until now, just a pipe dream for most sea-ice experts. It was always assumed that such an expedition would be too costly, the polar weather too unpredictable. But September 2019 saw the start of something that had long been considered impossible. Within the framework of the international MOSAiC expedition, the German research icebreaker Polarstern allowed itself to become trapped in the Arctic sea ice, offering researchers from 20 countries a once-in-a-lifetime opportunity. In a camp on the Central Arctic ice, around the clock they investigated the sea ice, ocean, atmosphere and life in the sea. They witnessed a dramatic transformation of the North Pole region, the consequences of which are likely to affect the sea ice first.