Entering the expansive Zócalo in Mexico City is a disorienting experience. At one end of this vast square stands the capital’s cathedral, its tall spires leaning slightly to one side. Adjacent to it, the Metropolitan Sanctuary leans in the opposite direction, while the National Palace appears to be tilting as well.
The noticeable tilt of these historic structures is a stark indication of a long-standing issue: Mexico City is experiencing significant subsidence. This alarming phenomenon has been ongoing for over a century.
Recent advancements have allowed for real-time monitoring of this subsidence through the use of one of the most advanced radar systems ever deployed into orbit. Named Nisar, this satellite can detect minute variations in the Earth’s surface, even in the presence of dense vegetation or cloud cover.
“Nisar elevates the capability of radar imaging of the Earth,” remarked Marin Govorčin, a scientist at NASA’s Jet Propulsion Laboratory. “It can detect any change, regardless of size, that occurs on Earth from one week to the next. No other imaging mission can achieve this level of detail.”
While this is not the first instance of observing Mexico City’s subsidence from space, the Nisar mission offers unprecedented insights into the extent and variability of the sinking across different land types. It has also successfully penetrated previously hard-to-reach areas on the outskirts of the city, which pose challenges due to complex topography.
The significance of this imagery extends well beyond Mexico City. “The findings related to Mexico City illustrate just a fraction of the potential applications that the Nisar system can facilitate,” stated Darío Solano-Rojas, an engineer at Mexico’s National Autonomous University (UNAM). “This includes not only the monitoring of sinking cities but also the study of volcanoes, earthquake-related deformations, and landslides.”
Nasa has indicated that this technology also has the capacity to monitor various environmental issues, including climate change, glacier movement, agricultural yield, soil moisture levels, forestry health, and coastal flooding.
“These initial images are merely the beginning,” commented David Bekaert, a project manager at the Flemish Institute for Technological Research and a member of the Nisar science team. “We can anticipate a surge of new findings from around the globe.”
The Nisar system, a collaboration between NASA and the Indian Space Research Organization, has revealed that certain regions of Mexico City, including areas surrounding the main airport, are sinking at rates exceeding 2 centimeters per month, ranking among the fastest subsidence rates globally.
A prominent example of this rapid decline is the Angel of Independence statue located on Paseo de la Reforma, the city’s main avenue. Erected in 1910 to celebrate a century of Mexican independence, the 36-meter monument has required the addition of 14 steps to its base due to the gradual sinking of the land surrounding it.
The effects of Mexico City’s subsidence are widespread, impacting the lives of approximately 22 million residents. This includes leaning buildings, distorted roads, and damage to the underground metro infrastructure.
Efraín Ovando Shelley, another engineer at UNAM, explained, “It impacts the entire urban infrastructure, including streets, water distribution pipes, water supply, and drainage systems.”
The phenomenon was first recorded in 1925 and is attributed to centuries of groundwater extraction. Built on an ancient lakebed, the soil beneath Mexico City is particularly soft. The extraction of water from the underlying aquifer causes the clay-like earth to compress, leading to the gradual sinking of the city.
Govorčin further elaborated, “The primary cause of Mexico City’s subsidence is the excessive pumping of groundwater from the aquifer below, which surpasses the natural replenishment from precipitation. Withdrawing water causes the aquifer to compact under the weight of the city above.”
The underground aquifer supplies about half of the city’s water. However, as extraction rates have increased, the aquifer has shrunk significantly, with the water table dropping by approximately 40 centimeters each year.
This creates a detrimental cycle: as the city sinks, aging pipes that distribute water become damaged and leak, resulting in an estimated 40% loss of water. Compounding this issue is the ongoing climate crisis, which has led to several years of low rainfall, placing the metropolis at risk of a critical water shortage.
While there have been minimal efforts to address the issue beyond reinforcing the foundations of historic buildings, experts believe the Nisar imagery will draw increased attention to the situation. Nonetheless, halting the subsidence presents significant challenges.
