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Oceanographer Chris German explains to presenter Chhavi Sachdev something that had stumped scientists – why the seas didn’t keep getting saltier and saltier. Working backwards from a small discovery with seawater and Epsom salts (magnesium sulphate) led to a major discovery in the 1970s.

(Photo: A grey smoker of the Panarea volcanic island in the Mediterranean Sea. Credit: Alexis Rosenfeld/Getty Images)