The world's longest cave system

Believe it or not, the longest known cave system in the world is in Mammoth Cave National Park in central Kentucky, about 85 miles from Louisville and Nashville. For a Midwesterner with national park aspirations, this is the closest I can get without getting on a plane.

In its first 15 years as a national park, only 40 miles of passageways had been mapped out at Mammoth Cave. Today, over 365 miles of passageways have been surveyed, although some geologists think there could be several hundred more waiting to be discovered.

The caves reside within layers of limestone and sandstone. The caves are constantly shaped by rainwater that travels below ground. Over time, water erodes the limestone and creates streams that eventually carve out new passageways.

For those who crave solace away from the crowd, you'll be disappointed. You can't see these caves without signing up for a tour. But go anyway, because it's fascinating.

We were there to spend a night in the backcountry, but we also signed up for a couple of tours. The Snowball Tour leads you to descend 200 stairs, pass a massive rock pile called the Rocky Mountains, and enter an elliptical-shaped passageway encrusted with white gypsum crystals. The gypsum crystals, we were told by the tour guides, indicated a dry cave. Aside from the steep stairs, the tour was otherwise flat and easy.

On the other hand, the Historic Tour was far more interesting. It's called historic because of its role in U.S. history - most of the saltpeter used in the War of 1812 (between the U.S. and British Empire) was mined here. This wet cave tour had double the stairs, and more diversity in the cave interior - you go from being in a "mammoth" domed passageway (carved by a river) to squeezing yourself into a tiny slot called the Fat Man's Misery.

There are plenty of tours to choose from depending on how much time and energy you have. The adventurous can don helmets, headlamps and knee pads to crawl and squeeze themselves through undeveloped passageways on the Introduction to Caving and Wild Cave Tours. (The Trog Tour is available for kids ages 8-12.)