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Mexico City is sinking around its historic Angel of Independence
This is the root of the unique problem.
An immense city has been built on an unstable lake bed.
To provide water for the millions, the aquifer - a vast water reserve - is being drained. As the water is sucked out, the city lurches downwards.
You do not have to wander far to see the results.
Pavements and roads are cracked.
The walls of buildings are buckled, with their balconies leaning at crazy angles.
The most extraordinary example is the symbol of the city: a monument celebrating Mexico's independence from Spain.
This towering stone column topped with a golden angel was built in the 1900s.
Recently, 23 new steps had to be added to reach its base as the city had sunk around it.
And it is going to carry on sinking.