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SubTropolis is a 55,000,000-square-foot (5,100,000 m2), 1,100-acre (4.5 km2) artificial cave in the bluffs above the Missouri River in Kansas City, Missouri, United States, that is claimed to be the world's largest underground storage facility. Developed by late Kansas City Chiefs owner Lamar Hunt via Hunt Midwest Real Estate Development, Inc., it has trademarked the phrase World's Largest Underground Business Complex.
|Location||Kansas City, Missouri, United States|
Dug into the Bethany Falls limestone mine, SubTropolis is, in places, 160 feet (49 m) beneath the surface. It has a grid of 16 ft (4.9 m) high, 40 ft (12 m) wide tunnels separated by 25 ft (7.6 m) square limestone pillars created by the room and pillar method of hard rock mining. The complex contains almost 7 miles (11 km) of illuminated, paved roads and several miles of railroad track. Currently, more than 6,000,000 square feet (560,000 m2) is occupied and 8,000,000 square feet (740,000 m2) are available for future expansion.
The mine naturally maintains temperatures between 65 and 70 °F (18 and 21 °C) year-round. The United States Postal Service and the United States Environmental Protection Agency lease spaces within SubTropolis, the United States Postal Service for its collectible stamp operations and the U.S. Environmental Protection Agency for their Region-7 Training and Logistics Center. The National Archives and Records Administration also leases space for a Federal Records Center. 
On the north edge of the complex Hunt developed the Worlds of Fun and Oceans of Fun amusement park complex. Hunt's extensive business dealings in Clay County contributed to the Chiefs having their NFL Training Camp at William Jewell College in Liberty, Missouri until 1991.
Other facilities like SubTropolis exist although not on the same scale, such as the abandoned mine in Butler, Pennsylvania used by Corbis and the US Federal Government for secure storage. As the room and pillar mining method is used to extract limestone throughout the Midwest, many companies are looking at ways to utilize the hundreds of millions of square feet created in this manner for everything from mushroom farming to crude oil stockpiling.