Shanghai’s skyline glitters with sleek financial skyscrapers and glossy residential towers, but below the city’s lustrous facade lies an enduring problem. Thanks to mass urban migration, soft soil and global warming, Shanghai is sinking, and has been for decades. Since 1921, China’s most populous city has descended more than 6 ft. Across China, land subsidence affects more than 50 cities, where 49,000 sq. mi. of land have dropped at least 8 in.