Exhalation causes the chest to contract.

Most people do not attach great importance to this fact. But imagine that you are belly-crawling through a 10-inch-tall opening in a cave passageway, 250 feet underground. Your face is flat against the clammy limestone floor, and your legs are frantically trying to force your rib cage through a space where it does not seem to fit.

In this context, the physiology of breathing seems quite important indeed.

I know. In this position during the Wild Cave Tour at Kentucky's Mammoth Cave National Park, I remembered the advice of my guide: Exhale. The extra centimeters allowed me to squirm forward and continue my crawling tour of the world's longest cave.

With about 365 miles of explored passageways -- and counting -- Mammoth Cave is one of the nation's oldest tourist attractions. It began luring visitors just after the War of 1812, when it was mined for saltpeter, one of the raw materials for gunpowder.

Its charms were obvious: passageways the width of boulevards, underground rivers, towering vertical shafts and concert-hall-size chambers. Often led by slaves, early tours attracted European adventurers, famous actors and even transcendentalist writer Ralph Waldo Emerson, who mentioned the cave in an essay. A national park since 1941, Mammoth ranks with New Mexico's Carlsbad Caverns and South Dakota's Jewel Cave among the country's most impressive underground attractions.

I visited the park for five days in early June, ready to rediscover my muscles and uncross my eyes after my last semester of graduate school. I'd never been to Mammoth and had only a murky sense that crawling through its tunnels might be cool.

But wait a second. Can't cave tours be a little, well, hokey? I think of cheesy advertisements on billboards in cow pastures. Fanny-packed families gawking at stalactites. Gift shops with geodes.

The drive to Mammoth does nothing to alleviate these concerns. The interstate exit for the park is guarded by a large, tiger-striped T. rex, an ad for an attraction featuring life-size depictions of dinosaurs. Drive farther toward the cave and you'll pass a variety of miniature golf courses, a wax museum and at least a couple souvenir rock shops -- offering geodes.

But the kitsch ends where the national park begins. Ranger-led tours offer nothing more gimmicky than history and geology, and many involve moderately long walks and hundreds of stairs.

And for those seeking a bit more adventure, there is another option still. But it involves crouching. Then crawling. Then slithering through muddy tunnels like an earthworm. In fact, a week after taking it, I still had the scabs and ruined jeans to prove it -- the park's Wild Cave Tour is no joke.

Mammoth Cave is spectacular, but the national park is more than a monumental hole in the ground.

In addition to the cave itself, Mammoth offers 52,830 acres of above-ground activities, including hiking, paddling and fishing. In particular, the north part of the park is crisscrossed by more than 70 miles of backcountry trails, all of them lined with oaks, maples, poplars and other residents of the Eastern deciduous forest. The Green and Nolin rivers also wind through the park for 31 collective miles of waterway.

Paddling the Green in June, my girlfriend, Laura, and I could sometimes spot low-hanging fog at water's edge, a marker of where cool cave water joined warmer surface water. Laura pointed out one such patch early in our trip, and we followed it into the mouth of a small, dead-end cave. Our dog, a liver-spotted Dalmatian named Bella, was on her first canoe trip. She began shivering in terror as we entered the cave, but the little kid in me felt the excitement of discovering a river hide-out. We looked out from the darkness onto the river flowing past, our voices echoing in the dank, chilly air.

Outside again, we paddled past cigar-shaped islands jutting out of the river like battleships. Around us, a handful of people fished from kayaks or drifted aimlessly. I lay back against the canoe's gunwale in a light mist, alternately closing my eyes and gazing at the enormous sycamores arching over the water. It was a lazy, easy, beautiful paddle.

Laura and I were already somewhat familiar with the Green, having pitched our tent in Houchins Ferry Campground on the river's banks. One of three car-accessible campgrounds in the park, Houchins Ferry attracts families wanting to car-camp and fish. When not fishing, their children ran along the grassy riverbank, often playing with Bella, who pranced around them.

But Mammoth's surface activities do not all involve water. Laura, Bella and I also spent a day on the deserted trails looping through the park's northern half, hiking through the shafts of sunlight that penetrated the canopy of maples, oaks and beeches. We saw a grand total of one other person. And even on the ridges, there were hints of the realm below. We paused beside a spring that emerged from the hillside, then trickled over a small, moss-draped waterfall.

Then we looked into the damp pit of a sinkhole, wondering what worlds lay beneath our feet.

Prehistoric tribes first ventured inside the cave about 4,000 years ago, and it is easy to imagine how they might have found the entrance. On a sweltering summer day, the cool underground air pouring out of the "historic entrance" feels like a blast from an enormous air conditioner.

Following the cool draft inside, modern visitors quickly find the ruins of the old saltpeter mines. A bit deeper lie small huts that once belonged to a tuberculosis hospital -- a 19th-century venture that proved better at killing patients than curing them. Another chamber once hosted lantern-light church services.

Along with the sheer size and extent of its chambers, this rich history sets Mammoth apart from other tourist caves. Rather than merely holding forth on geology, guides tell of the African American slaves who served as early tour guides and who rank among the cave's most intrepid explorers. They mention the discovery of ancient mummies, as well as entrepreneurs who searched for new cave entrances by setting off underground charges of dynamite.

For geological reasons, Mammoth has relatively few classic cave formations -- stalactites, stalagmites and the like. But it does have some, and its history and scale easily make up for their scarcity.

Of course, no one tour can include more than a sliver of the world's longest cave. Besides the Wild Cave Tour (which begins miles from the historic entrance), I took the Historic Tour, which covers part of the early tourist route. The trip included mining ruins, a half-acre-size chamber and 440 stairs ascending a 200-foot-tall underground shaft, where water from an outside thunderstorm gave me a quick shower.

Although I sometimes had to crouch on this tour, often I could luxuriate in chambers the size of airplane hangars. Of course, some people won't consider a cave chamber, no matter how spacious, as grounds for luxuriating. I can only say that these people have never been on the Wild Cave Tour.

Think of an enormous performing arts center -- grand halls, beautiful auditoriums and some good artwork on the walls. Now imagine exploring it via the air conditioning ducts. This should give you an idea of what Mammoth's Wild Cave Tour is all about.

Several caves around the country offer such tours, and I had taken an earlier -- and much easier -- one in Arkansas a year before visiting Mammoth. Both were fun, and they probably qualified me as a spelunker. But not as a caver.

"Cavers" are a relatively small group of skilled adventurers, somewhat akin to serious mountain climbers, though not nearly as celebrated. They explore the most remote parts of caves, sometimes mapping what they find.

Spelunkers?

"Cavers rescue spelunkers," smirked Kevin Neff, our wry 62-year-old guide.

My group of five -- Bella stayed behind in a kennel -- included no one better than a spelunker. Still, Neff knew we hadn't come to dodder along the tourist trail. Just after entering the cave, he gave us a moment to adjust our hard hats and strap on our kneepads. Then he looked down his nose and feigned a cave connoisseur's disdain for us dilettantes.

"I'm assuming you're ready," he said dryly. "Because we're leaving."

The crawling began almost immediately. As we veered off the main tourist trail, the shrinking passages forced us to our knees. Our hands smacked the clammy cave floor, then sank into soft powder and finally clawed at the edges of the Bare Hole, the tiny cataract where I had to exhale.

This was a test, Neff explained. Although we could all physically fit through the hole (people with chest or hip measurements larger than 42 inches cannot take the tour), the squeeze is more than some people can stomach mentally. Closet claustrophobics will be outed.

But we all passed, and the tour continued in a blur of crawling. I lurched and gasped through pass-ages with names like Birth Canal, all while listening to the nonstop monologue of the man behind me, who repeated things like "That was easy!" and "Piece o' cake! Piece o' cake!"

I crawl like an ostrich, so I was pleased that the second half of the tour involved upright walking, as well as a bit of free climbing down tall crevices and cave walls. The highlight, though, came when Neff began to make eerie "Star Trek"-style "oohing" noises, which disappeared into the vastness of a room called Cathedral Domes.

Here, the cave's ceiling soared some 100 feet above our heads, forming a vaulted chamber maybe 40 yards long. The white light of our headlamps sliced through the blackness to the ceiling, lighting the clouds of our breath as they drifted upward.

Eventually, though, the passageways shrank and, alas, crawling resumed.

It wasn't pretty. I struggled over the sharp, irregular rocks that jutted from the floor of the Cheese Grater. I tried to straddle the puddles at the bottom of the Sewer Pipe. Then we came to Shotgun, which Neff said would certify us as real cavers.

Others navigated this tunnel by wedging their elbows against its edges and splaying their legs over the orange water at its base. They then crawled forward, bellies hovering over the water.

Right.

I collapsed and splashed straight through, sending a torrent gushing out the tunnel's end. After I emerged, Neff commented that he'd gotten the edge of his bag wet. My T-shirt plastered to my chest, I said I felt really, truly sorry for him.

Finally, the cave spit us back onto the main tourist trail, and we started walking toward the exit. Just before hitting daylight, we passed a regular tour group, stopped for a ranger talk in one of the cave's biggest passageways.

Undoubtedly they were impressed by such geological wonders. But walking by, I wondered what they thought of me -- wearing a hard hat, sopping wet and covered in cave dirt. Maybe, just maybe, they thought something flattering but completely untrue.

Maybe they thought they'd just seen a real caver.

Atlanta freelancer Ben Brazil last wrote for Travel about his short-lived modeling career in Argentina.

At Mammoth Cave National Park in Kentucky, the Wild Cave Tour covers a small section of the 365-mile underground system. On the tour, adventurers climb through tight, dark spaces on hands, knees and even bellies and twist down narrow openings.At Mammoth Cave National Park in Kentucky, visitors can enjoy activities both above and below ground. Above, Laura Ricca paddles out of a small, dead-end cave along the Green River.The author squeezes through a tight spot on the Wild Cave Tour, which takes visitors to the most remote parts of the park's underground maze. A park ranger shows off the tall, interconnected domes below Mammoth Cave's New Entrance.