Cold, dense melt water is sinking to the bottom of the ocean. That affects the salt content and aeration of the deeper parts of the ocean stretching right up to the equator. But the intensity of this vast oceanic cooling system has risen and fallen over the past few years. Researchers on board the German research ship "Polarstern" hope to discover whether the fluctuations are normal and have always been around, or whether they're connected to global warming.