Oklahoma is the 20th-largest state in the United States, covering an area of 69,898 square miles, with 68,667 square miles of land and 1,231 square miles of water. It is one of six states on the Frontier Strip, and lies partly in the Great Plains near the geographical center of the 48 contiguous states. It is bounded on the east by Arkansas and Missouri, on the north by Kansas, on the northwest by Colorado, on the far west by New Mexico, and on the south and near-west by Texas.
The state has four primary mountain ranges: the Ouachita Mountains, the Arbuckle Mountains, the Wichita Mountains, and the Ozark Mountains. The U.S. Interior Highlands Region, which contains the Ozark and Ouachita Mountains, is the only major mountainous region between the Rocky Mountains and the Appalachians. A portion of the Flint Hills stretches into north-central Oklahoma, and in the state's southeastern corner, Cavanal Hill is officially regarded as the world's tallest hill; at 1,999 feet (609 m), it fails the definition of a mountain by one foot. More than 500 named creeks and rivers make up Oklahoma's waterways, and with 200 lakes created by dams, it holds the highest number of artificial reservoirs in the nation.
Among the most geographically diverse states, Oklahoma is one of four to harbor more than 10 distinct ecological regions, containing eleven within its borders, more per square mile than in any other state by a wide margin. Marked by differences in geographical diversity between its western and eastern halves, eastern Oklahoma touches eight ecological regions, while its western half holds three.