Kentucky is located in the east central U.S. and is normally included in the group of southern states. Kentucky is one of four states constituted as a commonwealth. Known as the "Bluegrass State," it is a land with diverse environments and abundant resources, including the world's longest cave system. The metropolitan areas of Louisville, Lexington, and Northern Kentucky have a combined population of 2,169,394, which is 51% of the state's total population. The two other rapidly growing urban areas in Kentucky are the Bowling Green area and the Tri-Cities Region of southeastern Kentucky.