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In 1975, San Diego Zoo began placing tissue samples of rare animals in cryogenic storage for the benefit of future generations. Called the Frozen Zoo, the refrigeration system now contains the cells of more than 1000 species ranging from the white rhinoceros to the black-footed ferret. Scientists are now using the collection to try to save species threatened by extinction. Simon Watts talks to Dr Oliver Ryder, who has worked at the Frozen Zoo from the very beginning. PHOTO: Northern White Rhino cells in the Frozen Zoo (San Diego Zoo Institute For Conservation Research)