Researchers mapped out how land is moving vertically across the 28 most populous U.S. cities and found all the cities were compressing like a deflated air mattress to some extent. Twenty-five of them are dropping across two-thirds of their land. About 34 million people — about 10 percent of the U.S. population — live in the subsiding areas, according to the study published Thursday in Nature Cities. It might not seem like an issue now, but when the foundation of a big building is sinking in the middle of the city, the structural integrity is probably not great.