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Coral reefs are sometimes called the rainforests of the sea because of the extraordinary variety of life they support. The reefs themselves are built by tiny animals called polyps, which extract minerals from seawater to construct hard skeletons over many generations. When ocean temperatures rise too high, the colourful algae living inside the polyps are expelled, leaving the coral pale and weakened in a process known as bleaching. Bleached coral is not necessarily dead, and if conditions improve quickly enough, the algae can return. Prolonged stress, however, often proves fatal, and scientists warn that warming seas are making such events increasingly common.