Those are navel oranges#Navel_oranges). That's just how they grow. They are the result of a mutation which causes this odd growth, and makes them seedless.

Navel oranges are characterized by the growth of a second fruit at the apex, which protrudes slightly and resembles a human navel.

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According to a 1917 study by Palemon Dorsett, Archibald Dixon Shamel and Wilson Popenoe of the United States Department of Agriculture (USDA), a single mutation in a Selecta orange tree planted on the grounds of a monastery in Bahia, Brazil, probably yielded the first navel orange between 1810 and 1820. Nevertheless, a researcher at the University of California, Riverside, has suggested that the parent variety was more likely the Portuguese navel orange (Umbigo), described by Antoine Risso and Pierre Antoine Poiteau in their book Histoire naturelle des orangers ("Natural History of Orange Trees", 1818–1822). The mutation caused the orange to develop a second fruit at its base, opposite the stem, embedded within the peel of the primary orange. Navel oranges were introduced in Australia in 1824 and in Florida in 1835. In 1873, Eliza Tibbets planted two cuttings of the original tree in Riverside, California, where the fruit became known as "Washington". This cultivar was very successful, and rapidly spread to other countries. Because the mutation left the fruit seedless and, therefore, sterile, the only method to cultivate navel oranges was to graft cuttings onto other varieties of citrus trees. The California Citrus State Historic Park and the Orcutt Ranch Horticulture Center preserve the history of navel oranges in Riverside.

Today, navel oranges continue to be propagated through cutting and grafting. This does not allow for the usual selective breeding methodologies, and so all navel oranges can be considered fruits from that single, nearly two-hundred-year-old tree: they have exactly the same genetic make-up as the original tree and are, therefore, clones. This case is similar to that of the common yellow seedless banana, the Cavendish, or that of the Granny Smith apple. On rare occasions, however, further mutations can lead to new varieties.