(CNN) — There are two obvious attractions in Louisville, Kentucky: Bourbon and horses. Both are in fine form during the Kentucky Derby.
The Derby is the first race of the elusive Triple Crown. The race alone draws around 170,000 people to the track at Churchill Downs and 1.5 million to the city, adding to the nearly 16.5 million visitors to Louisville each year, according to the Louisville Convention and Visitors Bureau.
But when horse racing isn't in season, there's another far more mysterious attraction worth checking out in the city. Hidden under the Louisville Zoo, the Mega Cavern, once a limestone quarry, houses the only underground zip line in the United States.
And it's got several more secrets:
The federal government rents space in the Mega Cavern to house documents, according to the cavern's owner.
"We're in the process of building a new warehouse for some of those now that have walls that you can literally take a tractor trailer and drive into them and they won't go. Very, very heavy duty, blast-proof walls," said Tom Tyler, Mega Cavern's co-owner.
The cavern is also tornado and hurricane proof so any important documents won't get damaged. I tried to get Tyler to point them out, but he said they're secret.
Utilities are cheaper underground, and that's important when you are running a 100 acre operation. Utility costs are 75% to 85% cheaper than building above ground.
"We have a constant temperature of 58 degrees," said Tyler. That means when they need to crank up the heat, they only need to go up about 12 degrees. And the natural 58 degrees makes for chilly air conditioning.
Booze and chocolate
Need I say more. The cool temperature in the cave is ideal for storing alcohol and chocolate.
Rumor has it (according to my guide) that many local distilleries store bourbon in the cave. It's also rumored that the cavern houses Hershey's chocolate.
"I'm not entirely sure where it is, but it's around here somewhere," said my zipline guide Max Voirol, alluding to the dozens of corridors.
It was a bunker during the Cuban missile crisis
The Cuban missile crisis had America on edge. We were on the brink of nuclear war with the Soviets and Americans were preparing for the worst.
"In the 1960's this was scheduled to be a fallout shelter," said Voirol. "They had plans to store a whole bunch of people down here during the Cuban missile crisis. It was actually going to be the largest civil defense shelter in the entire country. Louisville, Kentucky would've been the place to be."
About 50,000 people could have fit in what is today the Mega Cavern. According to Voirol, at the time there were barracks, food supplies and a hospital facility set up in case the worst happened.
Some geologists say it's the safest place in Kentucky.