Standing in his office high above Latin America's largest city, the water board operations chief Alejandro Martinez smiles as he considers one of the ironies of Mexico City's development.
Five hundred years ago, it was a compact Aztec citadel set in a broad highland lagoon. But today it is a vast metropolis sprawling across a dried-up lake bed.
Mexico City's underlying aquifer is now collapsing at a staggering rate beneath the streets. While Venice slips into the Adriatic at a fraction of an inch each year, Mexico City is lurching downwards by as much as a foot a year in some areas. Over the past century, it has dropped 30ft.
Chugging the equivalent of one Olympic-sized swimming pool full of water every minute, the city's strained aquifers are dragging much of the capital's rich heritage down with them, while the 20 million residents face problems that include water-borne diseases, power outages and the threat of riots.
The result of a head-on collision between booming demand and finite resources, Mexico City provides a sneak preview of a situation that the United Nations warns could become widespread in coming decades as the world's mega-cities continue to grow unchecked and unplanned. Unesco claims that up to seven billion people from 60 countries could suffer water shortages by 2050.
Mr Martinez told The Independent: "The difficulties that we are confronting today could be faced by other cities in the future ... We have to look for new and alternative technologies to find a solution to the problem of producing water and avoid a crisis in the short term."
Once a thriving city of "chinampas", or floating gardens, linked to land by an elaborate system of causeways, the abundant water of Lake Texcoco was gradually drained to make way for the colonial capital after the Spanish Conquest in 1519. Despite rapid growth, the city continued to meet its water needs in the 19th century from springs, shallow wells and remaining surface water.
The first strains began to show with massive migration in the 1940s; the capital began swallowing up one satellite town after another as it grew by 7 per cent a year. Faced with shortfalls as the underlying sand and clay aquifers failed to keep pace with demand, city authorities tapped into two neighbouring river systems at a massive cost.
The city now has five pumping stations working around the clock to draw water vertically three-quarters of a mile from the neighbouring Cutzamala River basin and from the lower catchment area of the River Lerma. Paying about $50,000 (£28,000) a day in water rights alone, the system consumes the same amount of electricity as Puebla, a city of 1.3 million people to the south-east.
Now comprising 350 neighbourhoods packed into a smog-wreathed metropolitan area more than twice the size of greater London, the city swills a massive 10.5 million gallons of water each day.
Used by residentsand by water-intensive industries such as beer brewing and soft-drink bottling, the ever-expanding metropolis's supplies are again running short.
In several shanty towns on the outskirts, a growing army of bucket-wielding residents are forced to queue for water from a fleet of tanker trucks that fan out across the city each day as authorities admit to a growing shortfall. Mr. Martinez said: "In the past eight years, the supply of water has remained constant, while the population has grown. The network currently fails to reach about 2 per cent of the city, mostly in outlying areas on higher ground."
In the leafy park surrounding the imposing Monument to the Revolution, there is an old cast-iron well casing that has continued to hold as the city around it has sunk. Once flush with street level, the plain black pillar now stands 26ft high, serving as an unusual photograph stop for slack-jawed tourists.
Inching through cross-town traffic to the Avenida de la Reforma - a central thoroughfare that cuts through the city's upscale business district - a towering column topped with a golden angel comes into view. Built in the early 1900s to celebrate the centenary of the Mexican War of Independence, 23 steps were recently added to reach its base as the city fell away around it.
A short ride away in the north of the city, the yellow-domed Basilica of Our Lady of Guadalupe is also in trouble. Built more than two centuries ago to honour the patron saint of New Spain, the shrine tilted so heavily beneath the weight of millions of pilgrims that it was declared unsafe in the 1970s and a new basilica was built next to it. The listed building now serves as a museum.
But collapsing heritage is just the tip of the iceberg. Below street level, the ongoing subsidence is wreaking havoc with the water distribution and drainage systems. The city's 8,300-mile network of water pipes routinely fracture, losing up to 40 per cent of potable water supplies, according to some estimates. The city's sewage used to drain away by gravity towards a far-off outflow in the Gulf of Mexico but now needs to be first pumped uphill before it can be drained.