The Arctic Ocean was covered by up to 900 m thick shelf ice and was filled entirely with freshwater at least twice in the last 150,000 years. This surprising finding is the result of long-term research. With a detailed analysis of the composition of marine deposits, the scientists could demonstrate that the Arctic Ocean as well as the Nordic Seas did not contain sea-salt in at least two glacial periods. Instead, these oceans were filled with large amounts of freshwater under a thick ice shield. This water could then be released into the North Atlantic in very short periods of time. Such sudden freshwater inputs could explain rapid climate oscillations for which no satisfying explanation had been previously found.