Mexico City is slowly and inexorably sinking

The phenomenon, which is called "subsidence," could destroy structures on the surface: Mexico City is slowly and inexorably sinking.

With its 8.6 million inhabitants, Mexico City is the 21st most populous city in the world. Still speaking of rankings, it's also the first city in the world for the speed with which it's sinking into the ground.

The way these two numbers come together already gives a glimpse of the tragedy that's about to slowly unfold: over the next century and a half, some areas of the Mexican capital will sink as much as 19 meters, accentuating the irregular pattern that's already possible to see from above, as if homes and roads were built on waves.

Why and what it means that Mexico City is sinking

A study by the National Autonomous University of Mexico helps translate into numbers the imperceptible downward movement that's happening right under Mexicans' feet. It has been calculated that the ground of some neighborhoods is each year 50 cm closer to the center of the Earth, while some areas, just outside the boundaries of the megalopolis, will sink more than 30 meters in the next 150 years.

The reason for this phenomenon is rooted in the colonial history of Mexico City, the ancient Aztec capital of Tenochtitlan in fact was erected on an island in the middle of a lake. With the arrival of the Spaniards and the massacre of the indigenous people, the waters all around were drained so that the city could expand.

Drainage meant that under the streets, plazas and buildings of the new Tenochtitlan remained a layer of clay sediment, which we can imagine as a pile of messy plates that let a lot of water flow through the spaces between surfaces.

Over time, however, the Mexicans began to draw the water in the subsoil as if from an aquifer, triggering a side effect: the plates in the metaphor above lined up as if stacked in a dish drainer, compacting the sedimentary layer and giving rise to a phenomenon called "subsidence."

Why the subsidence of Mexico City should worry us

The fact that the movement develops in a more pronounced manner in some parts of the city only aggravates the danger of the phenomenon: "The difference between the speed of subsidence puts artificial structures at risk, subjecting them to different degrees of stress," explained geophysicist Enrique Cabral-Cano, one of the researchers who launched the alarm. In the long term, therefore, it is possible that metro tunnels, sewer systems and roads could break down due to an ever-increasing gradient. But not only public facilities are in danger, the risk also affects private homes, which in Mexico City are in very small part insured.

Unfortunately, by the scientists' own admission, the subsidence (which is not a phenomenon limited to the capital of Mexico) is not reversible: pumping water there where it has been extracted brings far too little result, nor is there any guarantee that stopping to draw from this precious underground resource can stop the birth of deeper and deeper depressions.

Maybe technology will find a solution in the next few years, in the meantime amazing innovations are already available to obtain (potable) water. Moreover the subsidence is not the only problem of the Central American country, of course, Netflix reminds us of it with this TV series that you probably know.

Giuseppe Giordano