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A 2004 scientific drilling expedition in the Arctic found an abundance of Azolla fern spores in samples of 50-million-year-old material from the seafloor. Because the Azolla fern can grow only in freshwater, some scientists believe the spores originated in river deltas or lagoons and drifted into the Arctic Ocean. However, Brinkhuis believes that the fern grew in a once-verdant ocean. The sheer abundance of the spores makes it unlikely, by some accounts, that they would have originated elsewhere. It is possible that geologic forces at one point closed the straits connecting other oceans to the Arctic Ocean, so water from rivers feeding into it pooled and formed a surface layer of freshwater where the fern could survive. A later opening of the straits would have wiped out the fern.