Montana ( mon-TAN-ə) is a landlocked state in the Mountain West subregion of the Western United States. It borders Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, and the Canadian provinces of Alberta, British Columbia, and Saskatchewan to the north. It is the fourth-largest state by area, the eighth-least populous state, and the third-least densely populated state. Its capital is Helena, while the most populous city is Billings. The western half of the state contains numerous mountain ranges, while the eastern half is characterized by western prairie terrain and badlands, with smaller mountain ranges found throughout the state. Most of Montana first came under American sovereignty with the Louisiana Purchase in 1803 and was explored by the Lewis and Clark Expedition shortly thereafter. Fur trappers followed and were the main economic activity in the area until gold was discovered in 1852. The ensuing gold rush, along with the passage of the Homestead Acts in 1862, brought large numbers of American settlers to Montana. Rapid population growth and development culminated in statehood on November 8, 1889. Mining, particularly around Butte and Helena, would remain the state's main economic engine through the mid-20th century. Montana has no official nickname but several unofficial ones, most notably "Big Sky Country", "The Treasure State", "Land of the Shining Mountains", and "The Last Best Place". Its economy is primarily based on agriculture, including ranching and cereal grain farming. Other significant economic resources include oil, gas, coal, mining, and lumber. The health care, service, defense, and government sectors are also significant to the state's economy. Montana's fastest-growing sector is tourism, with 12.6 million tourists (as of 2019) visiting the state each year.