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Scientists have discovered 'dark oxygen' being produced in the deep ocean, apparently by lumps of metal on the seafloor. About half the oxygen we breathe comes from the ocean. But, before this discovery, it was understood that it was made by marine plants photosynthesising - something that requires sunlight. Here, at depths of 5km, where no sunlight can penetrate, the oxygen appears to be produced by naturally occurring metallic 'nodules' which split seawater - H2O - into hydrogen and oxygen. So how significant is this discovery? The ��ѿ��ý's Science Correspondent, Victoria Gill, has the answers in this episode of our 5 Questions On podcast. Image Credit: Getty images