MEXICO CITY -- The majestic Metropolitan Cathedral was built as a symbol of the Spanish crown's domination over its new colonies in the Americas, showing the power of the Roman Catholic Church over pagan Aztec rituals.

The conquistadors built the cathedral right on top of the ruins of the pyramids they had systematically demolished after their conquest of Mexico in 1521.

Yet ever since, it has seemed as if the defeated Aztec warriors were fighting back -- reaching up from their graves to drag the most glorious icon of the capital of New Spain back into the earth. In this century alone, downtown Mexico City has sunk more than 25 feet as the water-starved ground has compressed; because some parts have sunk faster than others, one end of the cathedral settled nearly 8 feet deeper than the other.

As the ballooning Mexico City population devoured the capital's ground water throughout this century, the cathedral grew ever more lopsided, until it snapped in 1989 from the cumulative stresses, opening a dangerous crack across its spine. At that point, its gnarled naves and pillars were strapped with scaffolding and reinforcement beams.

Yet Mexican authorities never considered the prospect of failing to save the cathedral, which took 250 years to build and which art historian Manuel Toussaint described in 1944 as "the most notable monument of the colonial period in the Americas."

Today, the 435-year-old cathedral has nearly been righted, thanks to an ingenious rescue operation. Instead of needing last rites, the church will soon be healthy enough to throw away its structural supports and regain its original, unfettered splendor.

And in the process of saving the 127,000-ton, 400-foot-long cathedral, Mexican engineers have developed a method that offers hopes of rescue for crippled buildings around the world, including the Leaning Tower of Pisa.

Faced with budget constraints and the need to keep the cathedral functioning during the restoration, the team in charge of the project came up with a solution that was counterintuitive and comparatively cheap: Rather than try to prop up the sinking front end, the engineers would remove subsoil from beneath the other end -- and ever so carefully lower it back toward relative equilibrium.

"It is a process of observing and adjusting constantly," said architect Sergio Zaldivar, the project leader. "We are working against 400 years of deterioration."

Mexico City's varying sinking rates have caused damage to scores of historic buildings over the centuries, and frequent earthquakes have added to the structural havoc. Mexican engineers have built a reputation for their skill in shoring up stricken monuments -- though none posed such a challenge as the cathedral.

The heart of the cathedral's structural problem is that different sections of the building are sinking at widely different rates.

This bewildering range of settling rates led to dangerous buckling throughout the structure as well as cracking walls and roof vaults, causing the pillars to lean and creating waves in the floor that can make visitors dizzy.

Conquistador Hernando Cortes ordered that New Spain's first, humble cathedral be built in the center of the defeated Aztec capital, Tenochtitlan, in 1524. That church was replaced by the present cathedral, which was intended to be as grand as its towering counterparts in Seville and Granada, Spain.

Despite warnings from advisers that the ground was unstable, construction began in 1563. The church soon started to sink.

The builders had to alter the design several times and were forced to add more than 3 feet in length to the front facade to keep it in balance.

Most of the main work was completed by the 1770s, but changes were made until the final consecration ceremony in 1813, three years after Mexico declared its independence from Spain.

The new solution came from Enrique Tamez, an engineer who had used computer analysis to help lead the diagnostic procedure -- and who understood the complexity of the variegated earth under the cathedral and the contortions the structure was experiencing.

The engineering team dug 32 wells under the cathedral and the adjacent Sagrario church, built in the 1700s as a lateral extension of the cathedral. The concrete-lined wells are 10 feet in diameter and reach as far as 75 feet below ground. The wellheads are just below floor level and can be reached only through the eerie corridors of the cathedral's crypts.

At the bottom of each well, dozens of portholes give access to the muddy clay. Using hydraulic drivers, the crews push steel pipes horizontally deep into the clay. The pipes fill up with clay and then are slowly pulled back into the the well and hauled to the surface.

Zaldivar's team is on the verge of declaring the project a success and stopping the excavations, at least for the next few years.

The cost has been about $2.5 million a year, which Zaldivar notes is "less than a highway overpass" and was far cheaper than any of the other options considered -- such as installing 1,500 pilings up to 200 feet deep.

The Italian government, keenly interested for the potential application of the approach at Pisa, contributed a sophisticated monitoring system. Otherwise, the expertise and technology have all been Mexican.

Foreign specialists from areas with similar problems, including Germany and Brazil, have visited to study the concept and its execution, Zaldivar says.

The sinking of the city results from overuse of the ground water in the Valley of Mexico, especially during this century. Not only has the area's population grown to about 18 million, but the percentage of those people with access to running water has risen fast, hugely increasing consumption.

Seventy percent of the city's water comes from wells tapping the ground water, and the wells have lowered the water table perilously throughout the valley. The central city, which in the Aztec era was a lake bed, has subsided nearly 26 feet as the drier subsoil has compressed.

"This is an apocalyptic barbarity," Zaldivar says of the city's self-inflicted water problems. He has to deal with the consequences, not only for the cathedral but for many other monuments.

"In the Mexican mentality, the cathedral represents the Mother Church. It is the liturgical center, the seat of the archbishop," says the Rev. Ruben Avila, who has served here since 1985.

Surrounded by the 55 miles of scaffolding that shroud the interior of the church, priests celebrate Mass several times a day.

"The cathedral is now in intensive care, full of tubes and machinery, as if it were staying alive on a respirator," Avila says. "But, thank the Lord, the cathedral will soon be able to show its fine face once again."

Pub Date: 6/27/98