About 70% of our planet is covered in water, and the vast majority of that water is in the salty oceans. But in a paper published recently, scientists from Germany propose that the Arctic Ocean was - at a relatively recent few points in Earth's history - actually entirely freshwater. Rivers and meltwater flushed out the salt, and lower sea levels worldwide at the time meant it couldn't come back, at least for a while. Getting to grips with how and exactly when this was happening is critical to our understanding of how climate change affects the Arctic, as Katie Haylor heard from Walter... Like this podcast? Please help us by supporting the Naked Scientists