The interior of SubTropolis. SubTropolis is a 55,000,000 square foot (5,060,000 m³), 1,100-acre (4.5 km 2) 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.he 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..