If you’ve ever been to the Worlds of Fun Amusement Park in Kansas City, you probably didn’t know that you were standing about 150 feet ABOVE the largest business complex in North America. You’d be even less likely to know that on part of the business complex, formerly a massive salt mine, houses rare original reels of some of Hollywood’s greatest films including Gone With The Wind and The Wizard of Oz.

The complex is called the Hunt SubTropolis. Thousands of people go to work there each day via over 7 miles of underground, illuminated roads and several miles of railroad track. So, plenty of people go into the SubTropolis, workers and visitors alike. However, hardly ANYONE is allowed to go into the massive film repository. That’s the 45,000 square foot “locker” that contains hundreds of thousands of original spools of classic (and not so classic) films. Described as looking like “the warehouse scene from Raiders of the Lost Ark” the film industry and it’s constituent studios use the vault to hold and preserve all the reels of film that are the closest to the originals. There are millions of recorded copies of, say, The Wizard of Oz in the world. But there are only a handful of the original master spools. They want to keep these originals as safe and undamaged as possible because, as good as your TV picture is, a copy of a copy of a copy (times a thousand) isn’t going to be as good as a copy of the original. But, why in a salt mine.

The answer is: to prevent vinegar syndrome. That’s what it’s called when film is stored where the temperature is anything other than 45 degrees Fahrenheit and 25 percent humidity. Extreme cold isn’t good for film but any temperature over 45 degrees leads to vinegar syndrome…when the old celluloid starts to rot and smell like vinegar. The hotter the conditions, the less time it will take for vinegar syndrome to set in.

It’s not just celluloid reels, though. The U.S. Postal Service keeps their collection of rare stamps in a separate vault for many of the same reasons.

Also, to hedge their bets, most film companies keep the earliest versions of their films in two other underground vaults elsewhere in the United States. The thinking is, that if a freak earthquake destroys the Missouri facility there will still be an early copy of “Attack of the 50 Foot Woman” in the other facility in Pennsylvania. It’s all about priorities, people.

You can’t go see the actual SubTropolis Film vault; the public isn’t invited nor do they do private tours. In fact, they want to keep humans out of there as much as possible, using the old films only when they want to remaster them onto the latest technology. Here’s a brief (and very rare) filmed tour. The super-secret film locker is shown at the :42 s. mark.