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Overfishing of sharks has reduced their numbers at the top of the food chain and how this is causing other species valued as human seafood to become rarer. Semi-identical twins resulting from the double-fertilisation of the mother's egg. A piece of four-billion-year-old sea floor is discovered in Greenland. Endangered blue iguanas released on Grand Cayman in an attempt to save the species. There are more than 200 projects planned for International Polar Year. Plus, Antarctic explorer Charles Bentley recalls his surprise at discovering the Sentinel Range – and Mount Bentley – in 1958.