Each year, about 700,000 people take the short trip off Interstate 65 to visit Mammoth Cave, with the peak time coming in summer. (It's about a 12-hour drive from Philadelphia.) In winter, the cave's temperate climate gives it a balmy feel, said park spokeswoman Vickie Carson.
Tours cater to a range of visitors, from hardy adventurers to those looking for less-strenuous outings.
One tour suited for small children, the elderly, and others who can't walk long distances covers a quarter-mile and includes just a dozen stairs. Other tours cover four to five miles and put participants through vigorous workouts.
On the popular "Historic Tour," visitors take a winding two-mile hike that leads them 310 feet below the surface.
It's not a stroll in the park. Participants have to chug up and down 440 stairs. Wide walkways leading into expansive "rooms" give way to belly-scraping paths that take a serpentine route through the rock.
For adults, there's stooping to maneuver past low rock ceilings and sideways squeezing to slip through a narrow passage in a part of the cave known as "Fat Man's Misery." There's a steep climb up a stairway at a majestic part of the cave known as Mammoth Dome.
"How are those hour-long aerobic workouts working for you now?" Hyatt asked her mother as visitors huffed their way upward.
Dim lighting woven along points in the cave revealed a fascinating tapestry of rock formations during the two-hour excursion.
At one point, tour guide Nick Asher let visitors experience the cave in its purest form.
He told them that on the count of three, they would be enveloped in darkness. On count one, they should close their eyes, he coached them. On count two, he would cut off his lantern. On count three, they should open their eyes.
"And if you're afraid of the dark, just skip count three," Asher advised.
When group members opened their eyes, the only audible responses were "wow."
"This is total and complete darkness we're feeling down here," Asher said as visitors soaked in the feeling of being completely cut off from the world above. "And the cave is one of the only places on Earth where you can experience total silence."
After a few moments, he flicked a small lighter that illuminated the walls and ceiling.
Asher said that faint light was enough to get them back to the surface.
"Now when I say we, I mean Ranger Joe and I," he joked, referring to his sidekick, longtime tour guide Joe Duvall. "But we would surely send someone back to get you all later."
The experience was a highlight for Cameron Moreland, who fulfilled his boyhood dream with his visit to Mammoth Cave.
"That's exactly the kind of experience we wanted to get out of it," said Moreland, who took the tour with his wife and their three children from Green Bay, Wis.
The excursion was a big hit with his children — including the youngest, 6-year-old Cole.
"It was like a little boy's dream come true," the boy's father said of the adventure.
While the underground tours are the main attraction, there's plenty to do on the surface as well. The park also offers boating, fishing, horseback riding, camping, and walking trails amid the scenic rolling hills of south-central Kentucky.
The park is situated about 90 miles from both Louisville, Ky., and Nashville, Tenn.
There are about 400 caves underneath the park, but Mammoth Cave is by far the best known and longest — 392 miles of passageways have been mapped and surveyed.
Besides the underground sights, the visit provides a fascinating look at the layers of human history in a place that Asher calls a "time capsule," thanks to the temperate conditions that help keep things preserved.
"All the artifacts we're seeing are the originals," he said. "There are no reproductions down here."
Evidence indicates that the area's indigenous peoples ventured into the cave as far back as 2,000 to 4,000 years ago, apparently to scrape gypsum and other minerals off walls, said Carson, the park spokeswoman.
It wasn't until the 1790s that Mammoth Cave was found by white settlers. As the story goes, she said, a hunter shot at a bear that lumbered into the cave, with the man in hot pursuit.
For a time, the cave served as a valuable source of saltpeter, an ingredient in gunpowder. Mammoth Cave played a key role in supplying saltpeter for the young nation's military during the War of 1812. The remnants of saltpeter mining are still on display in the cave.
The first tours started in 1816, as the cave attracted wealthy visitors from the eastern United States and Europe.
Nearly two centuries later, the cave still fascinates throngs of sightseers making the trip to Kentucky.
"This is my idea of a vacation," Hyatt said as her tour group left the cave's natural air conditioning for the steamy temperatures on the surface. "It's something out of the ordinary. I went ... underground and got to see stuff that's been there for thousands of years."