SubTropolis

SubTropolis is a business complex located inside of a 55,000,000 ft2, 1100 acre artificial cave in the bluffs north of the Missouri River in Kansas City, Missouri, United States. It was developed by late Kansas City Chiefs owner Lamar Hunt via Hunt Midwest Real Estate Development, Inc., with the trademarked phrase World's Largest Underground Business Complex.

Dug into the Bethany Falls limestone mine, SubTropolis is up to 160 ft beneath the surface. It has a grid of 16 ft, 40 ft tunnels separated by 25 ft limestone pillars created by the room and pillar method of hard rock mining. The complex contains almost 10.5 mi of illuminated, paved roads and several miles of railroad track.

The mine naturally maintains year-round temperatures between 65 and 70 F. 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 its Region-7 Training and Logistics Center. The National Archives and Records Administration leases space for a Federal Records Center.

On the surface of the north edge of the complex, Hunt developed the Worlds of Fun and Oceans of Fun amusement park complex. His extensive business dealings in Clay County contributed to the Chiefs establishing an 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, such as for mushroom farming and crude oil stockpiling.