By SAM DILLON

The New York Times

MEXICO CITY - In a plaza next to one of this city's most important shrines, the colossal

monument to the Revolution, a humble water pipe has become a curious monument of its own to what is, literally, the collapse of Mexico City.

Flush with the ground in 1934 when the monument to the Revolution was built, the water pipe now juts 26 feet into the air.

Why? Firmly anchored in a hard layer of subsoil beneath the city's shallow aquifer, the pipe has stayed put in the last six decades while the city has fallen away.

Mexico City is sinking. So much water has been pumped out from the aquifer beneath it to satisfy the metropolitan area's 18 million residents that the ground is collapsing underfoot at a stunning rate.

Many cities have experienced subsidence. Venice, Italy, has sunk about nine inches during

the 20th century as its water table has dropped'. But from here, Venice's problems seem marginal. Mexico City has sunk 30 feet.

"The sinking of the soil in Mexico City is one of the biggest engineering problems any city has faced, anywhere," said Ismael Herrera Revilla, a mathematics professor at the National Autonomous University who led a scientific panel in a five-year study of the city's water crisis.

In 1519, when the Spaniards conquered the Aztec capital, Tenochtitlan, there was plenty of

Water; Mexico City originally straddled two lakes. But the conquistadors who built their own city next to the Aztec one brought engineers to drain the lakes.

decades of this century, annual sinkage in the city center averaged about two inches, but when it peaked at mid-century the soil was collapsing away at the astonishing rate of 19 inches a year.

Because the subsidence is not uniform, it has caused hundreds of millions of dollars in damage to buildings and other structures over the years, especially in the colonial-era city center.

Striking evidence of the sinking is visible on a subway line that runs above ground south of the city center. Horizontal when first constructed in the mid-1960s, the tracks now, look like a roller coaster.

After the subsidence shattered hundreds of irreplaceable colonial churches and mansions, the city stopped pumping water in the city center, instead drawing from wells on the periphery. In recent years, this has slowed the sinking of the city center to about an inch annually. But some suburbs with many wells continue to sink 18 to 24 inches each year.

Today, the most serious damage is occurring underground, where the collapsing subsoil continues to rupture sewer lines, subway tunnels and potable water pipes.

One recent day, Cesar Buenrostro, the city's new public works director, stepped onto an elevator at a construction site on Mexico City's eastern edge, descending 80 feet through a cement tube to show two reporters a giant drilling machine with 20-foot teeth that is chewing a new wastewater tunnel through the city's clay subsoil.

The tunnel is one of the last segments of a new 124-mile network of very deep sewers, designed to carry rain and waste out of the city. The $870 million system is needed not only to keep up with city growth, but also because sewage no longer flows by gravity into the Grand Drainage

Canal, which was for a century the main sewer outlet.

The city's 30-foot drop has forced the installation of vast pumping stations to elevate the sewage to the level of the canal entrance, greatly increasing operation costs. The new, deep sewer will reduce the reliance on. dumping to drain the city.

"Soil sinkage is a huge problem, but unfortunately we can' drastically reduce the pumping of the aquifer now, because the city needs the water," Buenrostro said. "Our basic problem I the concentration of politic: power, industry and most of our cultural treasures and educational institutions in the capita

We want to change that, but for now we have to provide water our people."