W alking into Mexico City’s sprawling central Zócalo 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 Govorčin, a scientist at Nasa’s jet propulsion laboratory. “Nisar will see any change big or small that happens on Earth from week to week.…