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The Africapolis regional database, which is derived from the Geopolis database, defines an ‘‘urban’’ agglomeration as a built-up area without more than 200 metres of discontinuity and with a minimum of 10,000 inhabitants. It is a geographical object that can be perceived in the material world. However, some agglomerations have population figures that reach those of large metropolises, and yet they are not accounted for in official national and international statistics. They are unidentified. These UGOs, Unidentified Geographical Objects, are characterised by the densification of the population in regions where traditional habitat was already dense. This article first describes the most remarkable UGOs in terms of size, and then provides explanations on how they emerged.