SubTropolis is a 55,000,000 square foot (5,060,000 m³), 1,100-acre (4.5 km2) manmade 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.
Dug into the Bethany Falls limestone mine SubTropolis is at 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 seven miles (11 km) of illuminated, paved roads and several miles of railroad track. Currently 5,000,000 square feet (460,000 m³) is occupied and 10,000,000 square feet (920,000 m³) are "improved." About 3.2 acres (13,000 m2) of available space are added each year as active mining continues.
The mine naturally maintains temperatures between 65 and 70 degrees Fahrenheit (18 to 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.