Kentucky, officially the Commonwealth of Kentucky, is a state located in the East Central United States of America. As classified by the United States Census Bureau, Kentucky is a Southern state, more specifically in the East South Central region. 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 fact that bluegrass is present in many of the pastures throughout the state, because of the fertile soil. It made possible the breeding of high-quality livestock, especially thoroughbred racing horses. 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. It 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, bluegrass music, automobile manufacturing, tobacco and college basketball.