Kentucky, officially the Commonwealth of Kentucky, is a state located in the east south-central region of the United States. Kentucky is one of four U.S. states constituted as a commonwealth (the others being Virginia, Pennsylvania, and Massachusetts). Originally a part of Virginia, in 1792 Kentucky became the 15th state to join the Union. Kentucky is the 37th most extensive and the 26th most populous of the 50 United States. Kentucky is known as the "Bluegrass State", a nickname based on the bluegrass found in many of its pastures because of the fertile soil. It is a land with diverse environments and abundant resources, including the world's longest cave system, Mammoth Cave National Park, the greatest length of navigable waterways and streams in the contiguous United States, and the two largest man-made lakes east of the Mississippi River. Kentucky is also home to the highest per capita number of deer and turkey in the United States, the largest free-ranging elk herd east of Montana, and the nation's most productive coalfield. Kentucky is also known for horse racing, bourbon distilleries, automobile manufacturing, tobacco, and college basketball.