Why are there lumps of metal at the bottom of the ocean?

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On the bottom of the ocean, there are millions of potato-sized lumps of metal called polymetallic nodules. They grow at a rate of one centimetre per million years and potentially contain every single element on the planet – providing a potential source of rare metals for the renewable energy industry. But how do these lumps of metal form? Prof Andrea Koschinsky from Jacobs University in Bremen tells Marnie Chesterton how it all works.