Sinking six inches annually, what is the world's fifth most populous city, with 21.6 million people?

Every year, the capital of Mexico sinks another six inches, as it was built on an island in Lake Texcoco. The Aztecs had used a network of dykes and canals to control flooding and supply water, but the Spanish colonists decided to drain the lake bed, which has caused this sinking. Mexico City has sunk a total of some 30 feet in the past century.

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