The Metropolitan Cathedral in Mexico City is the oldest of its kind in the Americas - but it's in crisis.

Peter Greste reports from inside and underneath the sinking cathedral

As the water under the structure is consumed by the city's 18 million inhabitants, the cathedral, which has been slowly sinking for hundreds of years, is now collapsing.

It is a spectacular monument to Catholicism in the heart of the world's biggest city, but like Italy's Leaning Tower of Pisa, the building has been sinking since the day it was built almost 500 years ago.

The problem is not so obvious from the outside, but for the clergy and the devoted Catholics who worship there, it is deeply disruptive.

Engineers have tried to limit the disruption

The team of architects and civil engineers who have worked to keep the building upright have done their best to limit the disruptions - they have managed to avoid closing it to the public, but it has been a major trial for the church's religious leaders.

Archbishop Padre Luis Avila Blancas says: "It is hard to express the feeling that working here evokes, because it involves people's faith.

"There's been centuries of co-operation to build this house of God. Even the artwork is from the 17th Century, which was a time when artists showed their love of God. This whole building is an expression of love and faith of all believers."

Amid the engineering works the cathedral remains the focus for thousands of Mexicans who depend on its religious support to help them through the country's social and economic troubles.

Thousands of Mexicans rely on the cathedral's support

Construction work began in 1536 - just 50 years after the Conquistadors first arrived here from Spain. That also makes it the very first Christian cathedral to be built anywhere in the Americas.

In a tunnel underneath the cathedral, archaeologists have found the building that originally stood on the site - the Aztec Pyramid of the Sun - their main temple of worship.

The Conquistadors chose to use it as the foundation for the Cathedral Sacrament, an audacious attempt to show just whose God was superior, and a breathtaking example of the clash between Aztec and European cultures.

The man who heads the team trying to save the building is Architect Sergio Saldivar. He has been studying it from every conceivable angle for the past decade knows the problems well.

The cathedral stands on Aztec remains

"The original Aztec city was a network of canals that earned the name of New Venice," he said. The ground is very soft and the Spanish didn't think it would sink, but by the time they'd added the roof to the south wing, it was already going down."

The water is still there beneath the metropolis but Mexico City's inhabitants are drinking the supply dry and the entire city, not just the Cathedral, is slowly collapsing.

The crisis is most obvious in the south wing - it has dipped almost a metre more than the rest of the structure.

But if the problem is underground, so is the solution. Workers began digging shafts under the cathedral in 1993 to try to straighten and level the building. Now they are slowly shoring up the sludge with shafts of concrete, to try to give the 127-thousand tonne edifice a solid base to rest on.

Workers have been digging shafts beneath the structure since 1993

Sergio Salvidar says: "The problem is improving the ground. We believe we have the technology to stabilise the building. It may need more work in the future, but we are confident that we have been able to save it."

One man with a front-row view of events is chief bell ringer Rafael Parra. Each day he treads the same route that five centuries of his predecessors have made to mark time across the city.

He says: "Tolling the main bell is a great pride - it fills me with a great spirituality. And it makes me aware that I am part of the history of this Cathedral."

"The restoration and maintenance work is vital for this colonial jewel. It is an edifice we cannot lose and it is Mexico's responsibility to protect and take care of it."