Research says that many major US cities are slowly sinking. Nashville is one of them. Other cities with the same issues include Chicago, Dallas, Denver, Houston, and New York. Some of the reasons are natural, and others are man-made. Now, this isn’t happening quickly. Nashville is sinking one millimeter per year. In other cities, the average is three millimeters per year. Researchers are trying to understand why and, more importantly, figure out what they can do to stop this. Some cities like Nashville are experiencing similar drops citywide. In other metropolitan areas, the dip can vary in different parts of town…