The average temperature in the Arctic is rising at a much faster rate than anywhere else on Earth. To understand why, the Alfred Wegener Institute organised MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate), the largest-ever scientific expedition to the Arctic. The in-situ data collected from the Arctic Ocean, atmosphere, sea ice, biogeochemistry and ecosystem by an international team of scientists during the yearlong ice drift onboard the German research icebreaker Polarstern will significantly advance research on warming in the Arctic and its global effects.