SubTropolis is a business complex located inside of a limestone mine in the bluffs north of the Missouri River in Kansas City, Missouri. 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.
SubTropolis is up to beneath the surface, dug into the Bethany Falls limestone mine. It has a grid of , tunnels separated by limestone pillars created by the room and pillar method of hard rock mining. The complex contains almost of illuminated, paved roads and several miles of railroad track.
The mine naturally maintains year-round temperatures between . The United States Postal Service and the U.S. Environmental Protection Agency lease spaces within SubTropolis, the U.S. Postal Service for its collectible stamp operations and the 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 a training camp site at Liberty's William Jewell College from 1963 until 1991.
Other facilities exist that are similar to SubTropolis, although not on the same scale, such as the abandoned mine in Butler, Pennsylvania used by Corbis and the Federal Government for secure storage. The room and pillar mining method is used to extract limestone throughout the Midwest, and many companies are looking at ways to utilize the hundreds of millions of square feet created in this manner, such as for mushroom farming or crude oil stockpiling.
The presence of vast quantities of limestone below Kansas City led to the development of expansive quarrying operations in the region from the mid 20th century onward. Sideways tunneling into the hills surrounding the city created an expansive network of underground chambers accessible at ground level between 12 and 15 feet in height, supported by pillars more than 24 feet wide. The first underground complex developed for commercial use opened in 1964, utilizing the tunnels and rooms for industrial storage and operations. Early tenants included Pillsbury and Russell Stover Candies. In the early 1970s, the Ford Motor Company leased 25 acres of the facility to store vehicle inventory.
SubTropolis was developed from limestone mines that supported Kansas City's construction in the late 19th and early 20th centuries. The site that now comprises SubTropolis began as a lateral mining operation east of present-day I-435 in Clay County, Missouri, by Midwest Precote, a regional asphalt producer. As mining activity decreased, the site was repurposed by Lamar Hunt, then-owner of the Dallas Texans and principal founder of the American Football League, Major League Soccer, and the Kansas City Chiefs. Hunt would become a chief investor in the Great Midwest Corporation, which purchased Midwest Precote in 1970 to develop emptied mining sites for commercial use. In 1973, Worlds of Fun amusement park was opened on a 300-acre site above the underground complex, receiving an expansion in 1982. 3 million square feet of commercial space had been developed by 1983, according to the company.
In 1985, Hunt Midwest Enterprises was formed from the merger of Great Midwest Corporation and Mid-America Enterprises, creating three subsidiaries, among them Hunt Midwest Mining, Inc., and Hunt Midwest Real Estate Development, Inc, which would come to handle the operation of SubTropolis's commercial holdings. The company sold Worlds of Fun and its expansion Oceans of Fun to Cedar Fair Entertainment Company in 1995, becoming a partial owner, and seating then-president and CEO Lee Derrough on its board.
In the fall of 2001, 50 employees at a postal facility in the complex were tested for anthrax after handling mail found to possess traces of the virus from Brentwood, Washington D.C., where several cases of anthrax poisoning had been reported, and 2 individuals had died of exposure. The workers were administered a course of antibiotics as a precaution, though no infection arose, and all further tests for the virus in facilities were negative.
The complex began with roughly 4 million acres, growing to encompass roughly 14 million acres by present day. Around the turn of the 21st century, roughly 4.3 million acres of mine space was being leased to 50 tenants, employing approximately 1,300 people. By 2024, the number employed exceeded 2,000.
Roughly a tenth of the complex is designated for commercial use, with mining operations continuing on the far edges of Hunt Midwest's holdings. It is also home to a United States Postal Service storage facility, opened in 1982, which houses and distributes copies of every stamp ever produced.