Colonel Ashley Chadbourne McKinley was an aerial photographer and explorer who, on November 29, 1929, rode aboard the first aircraft to fly over the South Pole alongside Admiral Richard Byrd. That experience with extreme cold shaped the rest of his career. When the Army Air Force designated Ladd Field in Fairbanks, Alaska, as its cold-weather testing facility in 1940, McKinley was stationed there and quickly recognized the problem: Alaskan weather was unpredictable. Sufficiently cold days were short and unreliable. His solution was audacious -- build a refrigerated hangar large enough to hold a bomber, somewhere the engineers could control conditions down to the degree. The Army chose Eglin Field in the Florida Panhandle, and construction began. Despite wartime material shortages and contractor strikes, the first tests ran in May 1947. On June 12, 1971, the facility was formally dedicated in McKinley's honor.

The numbers behind Building 440 are staggering. The main All-Weather Room stretches 252 feet wide, 201 feet deep, and 70 feet tall at its center -- large enough to swallow a C-5 Galaxy, one of the biggest aircraft in the U.S. inventory. The walls are insulated with glass-wool board sheathed in galvanized steel. The original floor consisted of reinforced-concrete slabs resting on cellular glass blocks, engineered to withstand both the crushing weight of aircraft and the thermal extremes they endure. Three massive refrigeration systems, originally built by the York Corporation, drive the cold. The facility also houses a Temperature-Altitude Chamber that can simulate conditions at extreme altitudes, and an Equipment Test Chamber used for tanks, trucks, and ground vehicles. Rain can pour at hurricane intensity. Snow falls on command. Wind howls through the chamber at gale force.

The list of machines that have endured the McKinley chamber reads like a history of American aviation. Early tests subjected the P-51 Mustang, P-38 Lightning, and B-29 Superfortress to arctic conditions. In subsequent decades, the F-22 Raptor, the Boeing 787 Dreamliner, and the Airbus A350 XWB all spent weeks inside Building 440, their systems stressed by temperatures no operational environment would likely exceed. The F-35 Lightning II completed its climatic testing in March 2015, walking the chamber down to minus 40 degrees while running test sequences along the way. It is not only military hardware that passes through. Private industry and other government agencies regularly use the facility. The chamber has frozen helicopters, baked satellites, and drenched missile systems. If it flies, drives, or launches, chances are it has been tested here.

The refrigeration technology inside Building 440 is itself a landmark. The American Society of Mechanical Engineers designated the McKinley Climatic Laboratory as a National Historic Mechanical Engineering Landmark, recognizing the ingenuity required to chill and heat such an enormous volume of air on demand. The original R-12 refrigerant system used a multi-stage compression process, with low-stage and high-stage compressors cycling liquid through cooling coils at pressures carefully calibrated to prevent vaporization. The original Allis-Chalmers induction motors have since been replaced by variable-frequency synchronous motors, and recent efforts have focused on replacing ozone-depleting refrigerants with modern alternatives. On October 6, 1997, the building was added to the National Register of Historic Places, a rare honor for a facility that remains fully operational.

There is something wonderfully absurd about the McKinley Climatic Laboratory's location. Outside Building 440, the Florida Panhandle bakes under subtropical sun, with summer temperatures regularly exceeding 90 degrees. Inside, engineers create conditions colder than anything experienced in Antarctica. The contrast captures the essence of the place: it exists to guarantee that the machines defending the nation will function anywhere on Earth, and the only way to prove that is to build the worst of every climate under one roof. More than seventy-five years after its first test, the McKinley Climatic Laboratory remains indispensable, the one building in the Department of Defense where winter, summer, desert, and monsoon all happen on the same Tuesday afternoon.