Mention a sinking city and it is a fair bet that Venice is the place which comes to mind, yet parts of the centre of Mexico City are sinking at an even faster rate than that of the fabled Italian lagoon city.
The Popocatepetl volcano erupted for the first time 23,000 years ago
Not so long ago, you used to be able to stand on the green wooded slopes of the Popocatepetl volcano and look down on Mexico City.
Now, the view is blocked by a dense brown cloud of pollution.
Peel back this vile smog, and you uncover one of the biggest megalopolises in the world.
In just half a century, its population has increased seven-fold.
Now, more than 20 million people live and work in this hectic, expanding mass of concrete.
The Mexican capital is host to an array of unenviable problems.
The gap between rich and poor is among the biggest in the world. The violent crime rate is steadily climbing.
More than four million cars clog the badly-planned and poorly-built streets.
Its toxic air causes breathing and skin disorders.
But, worse than any of this, it is sinking.
Like the world revolving, there is an imperceptible, but inexorable movement under way.
In this instance... it is downwards.
If you lift and soar from the side of the Popocatepetl volcano - down into the greasy smog - you may arrive at the Cibeles fountain, in a chic central district of the city.
Ringed by fashionable restaurants, it is a popular place for locals to come to relax.
The architect Jorge Legorreta often comes here for a lunchtime stroll. His greying hair is neatly brushed over his ears.
His thick moustache dances above a lively mouth. Jorge walks past a rusty steel pipe, about half a metre high.
He points at it. "Twenty-five years ago, this drain was level with the ground," he says, "but the whole area has just fallen away from around it."
Jorge's eyes twinkle as he remembers the city's illustrious past.
It was the jewel of the Aztec empire.
The Spanish conquistador, Hernan Cortes, first set eyes on Tenochtitlan, as it was then called in 1519.
One of his band of soldiers wondered whether it was a dream, calling it an "enchanted vision". It was a beautiful, well-planned floating city built in harmony with its surroundings, in the middle of a lake.
But it was not to stay that way for long.
The Spanish soon drained the water away, and started building.
"In the world and in the history of urbanisation," Jorge says, "there has never existed such a radical transformation."
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.
Ilan Adler, a lean, bespectacled environmental scientist, proudly shows off some large black plastic tanks in the back yard of his university buildings.
He has helped to devise a system that harvests rain, which now provides around 80% of the water used by the teachers and students.
But water conservation is not a high priority for those running Mexico City.
Up to 40% of the water sucked from the aquifer is wasted.
The creaking supply system is full of leaks.
In Ilan's words: "It's a very serious problem. It's estimated that in the past 100 years the city has sunk more than nine metres, which is the height of a three-storey building!
"It's mainly down to the over-extraction of water from the aquifer, and our lack of conservation."
This mismanagement has led to the greatest irony of all.
One in four people does not have access to piped fresh water.
Xochitl Gonzalez sighs as she turns the tap and nothing comes out.
She lives in Iztapalapa - one of the poorest areas of Mexico City - where houses made out of bare concrete blocks teeter on the sides of steep slopes.
People here have to rely on trucks which come once a week, carrying foul water which needs chlorination.
Sometimes they do not come at all.
She says: "When the water supply runs out, a decision has to be made whether to provide for schools, or for homes. When there isn't enough water to clean the schools, they have to close, because of the risk of infection."
Xochitl looks from her roof out across the grey, dirty expanse of the city.
She says: "To me, it just seems illogical. This city was built above a lake, and yet we don't have water."
From Our Own Correspondent was broadcast on Thursday, 8 December, 2005 at 1100 GMT on BBC Radio 4. Please check the programme schedules for World Service transmission times.