Stingrays swim in a shallow pool at the Brookfield Zoo in Brookfield, Illinois in this undated handout photo provided by Brookfield Zoo. The Brookfield Zoo near Chicago is investigating what caused all 54 of its stingrays to die, a zoo spokeswoman said on July 13, 2015. Four southern stingrays and 50 cownose rays died on Friday after the oxygen level dropped in the exhibit habitat, the zoo said in a statement. (Photo by Reuters/Brookfield Zoo)
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