BLIGHTED by pollution and crime, Mexico City is facing another problem — it is sinking into the ground, and fast.

This megalopolis of 22 million people is falling at an alarming rate, dropping by as much as 15 inches (38cm) a year in some areas, and by almost 30 feet over the past 100 years. The rate of collapse has accelerated as city authorities pump water from subterranean aquifers to quench the thirst of the spiralling population, which attracts another 1,000 migrants from the provinces each day.

Despite the efforts of preservationists, buildings are being torn apart. Inside Mexico City’s colonial-era cathedral, the tiled floor and vaulted roofs slant in opposite directions at crazy angles. Some parts have sunk almost 8ft, while others remained stable. To hold the place together, metal joists have been bolted to the vast stone structures, built from material taken from the demolished pyramids of the Aztecs.

“It’s a building in intensive therapy,” said Javier Cortés Rocha, one of the architects struggling to save the capital.

When Hernándo Cortéz and his conquistadors first set eyes on Tenochtitilan — what would later become Mexico City — in 1519, the Aztec city was a maze of islands built on a shallow lake. Having defeated the Aztecs in battle, the Spaniards destroyed their pyramids and turned them into huge palaces and cathedrals, draining the lake for urban expansion.

In the past 50 years, the population of the city has more than quadrupled. To keep up with demand, the city pumps 10,000 litres (2,200 gallons) of water per second out of the ground, hollowing out the subterranean lake that once kept the city in place.

The crisis brought Mexico some unwanted distinctions. The technology it has pioneered to shore up its crumbling monuments has already been exported to help to stabilise the Leaning Tower of Pisa. And this week, it hosts the World Water Forum to discuss ways in which water management will dominate the geopolitics of tomorrow.

“Water, if not properly managed, will start to become as bad an issue as oil,” said Ilan Adler, director of the International Renewable Resources Institute in Mexico.

Despite its pumping efforts, the city wastes about 40 per cent of the water through leaking pipes. The loss has created terrible shortages, with about one million residents left with no running water, dependant on tankers delivering often unsanitary supplies. Another seven to eight million people only receive water in their taps every few days.

Olga Hernández García, a housewife, says that she has running water on average on only one day out of five. In her yard in the impoverished suburb of Ixtapalapa there are buckets, plastic barrels and basins filled to the brim with water she has stored to keep her family of six going until the pipes flow again.

“We always have to be aware of the water so we can fill our basins,” she said while scrubbing clothes in a stone washbasin because her washingmachine uses too much water. Often when it does flow, it comes in the middle of the night. “I leave my window open so I can hear, and tell my husband, ‘The water’s on’. We get up and start gathering it.”

One of the few parts of the original Aztec city to survive is in Xochimilco, in the south. Here, boatmen punt along canals built among islands created by planting trees and plants on shallow mud banks. As the vegetation built up, islands emerged that could be used to raise corn. Xochimilco, despite its murky brown waters, is now a weekend getaway for Mexicans keen to retrace their past and enjoy a few beers on the water, accompanied by mariachi bands that float alongside. It almost did not make it, however — six years ago, it was nearly drained by the demands of the thirsty mega-city, and only government intervention stopped it disappearing completely.

SINKING FEELING

VENICE
Has sunk 1ft (30cm) in the past 300 years and is also threatened by rising sea

LAS VEGAS
Pumps fresh water from underground sources causing parts of the desert valley to sink up to 6ft between 1960 and 1990

SHANGHAI
Built on a swamp, parts of the city are sinking 1.5cm a year because of sky- scrapers built since the 1990s

BANGKOK
Sinking at the rate of up to 5cm a year. The entire city may be below sea level by 2050

NEW ORLEANS
Sunk 15ft (4.5m) since 1878, one of the highest rates of subsidence in the US

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