An anonymous reader quotes a report from the Guardian: Walking into Mexico City's sprawling central Zocalo is a dizzying experience. At one end of the plaza, the capital's cathedral, with its soaring spires, slumps in one direction. An attached church, known as the Metropolitan Sanctuary, tilts in the other. The nearby National Palace also seems off-kilter. The teetering of many of the capital's historic buildings is the most visible sign of a phenomenon that has been ongoing for more than a century: Mexico City is sinking at an alarming rate. Now, the metropolis's descent is being tracked in real time thanks to one of the most powerful radar systems ever launched into space. Known as Nisar, the satellite can detect minute changes in Earth's surface, even through thick vegetation or cloud cover. "Nisar takes radar imaging observations of Earth to the next level," said Marin Govorcin, a scientist at Nasa's jet propulsion laboratory. "Nisar will see any change big or small that happens on Earth from week to week. No other imaging mission can claim this."

Though not the first time that Mexico City's sinking has been observed from space, the Nisar mission has provided a greater sense of how far the sinking spreads and how it changes across different types of land than any other space-based sensor. It has also been able to penetrate areas on the outskirts of the city that were previously challenging to study because of the complex terrain. The implications of the imagery extend far beyond the Mexican capital. "This study of Mexico City speaks to the realm of possibilities that will open up thanks to the Nisar system," said Dario Solano-Rojas, an engineer at the National Autonomous University of Mexico (Unam). "And not just for sinking cities but also for studying volcanoes, for studying the deformation associated with earthquakes, for studying landslides." According to Nasa, the technology is also capable of monitoring the climate crisis, glacier sliding, agricultural productivity, soil moisture, forestry, coastal flooding and more. The Nisar system found that some parts of the city are dropping by more than 2cm a month. "First documented in 1925, the city's sinking is a result of centuries of exploitation of the groundwater," the report says. "Because Mexico City and its surrounds were built on an ancient lake bed, the soil beneath the city is extremely soft. When water is pumped out of the aquifer below, this clay-like earth compacts, resulting in a city that is quietly sinking."

The crisis is also self-reinforcing: as the city sinks, aging pipes crack and leak, causing Mexico City to lose an estimated 40% of its water, even as drought and climate change make supplies more fragile.

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