Near the halfway point of a trek along Mammoth Cave's Grand Avenue, park guide Gregg Bailey gathers our small group before turning off all the lights.

Plunged into total darkness and eerie silence, punctuated only by startled gasps, your first thought is, "This is cool!" Yes, and in more ways than one.

Ren Davis/Special (ENLARGE)
Woodland Cottages, some with up to four bedrooms, are among the accommodations at Mammoth Cave Hotel.
Ren Davis/Special (ENLARGE)
In 1926, President Calvin Coolidge signed legislation making Mammoth Cave a national park. Work on the park began in 1933, and it was dedicated in 1946.
Ren Davis/Special (ENLARGE)
A popular section of Mammoth Cave is called 'Frozen Niagara,' one of the few places in this central Kentucky landmark where visitors can see stalactites and stalagmites.
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IF YOU GO Getting there • Mammoth Cave National Park is a short distance from I-65, about 25 miles northeast of Bowling Green, Ky. Where to stay • Headquarters Campground (109 sites) and Houchins Ferry Primitive Campground (12 sites). Reservations: 1-800-365-2267, Web site. • Mammoth Cave Hotel, with 92 rooms ranging from motel-styleto woodland cottages. ($36-$82 daily). Reservations: 270-758-2225, Web site. Information • Mammoth Cave National Park: 270-758-2180; www.nps.gov/maca. There is a fee for tours and space is limited, so reservations are a must, especially in summer. Tickets are $4-$45 ($4 entry for self-guided tour, $46 for 6 1/2-hour Wild Cave Tour). For detailed tour information and reservations, 1-800-967-2283, reservations.nps.gov. Experienced cavers may contact the Cave Research Foundation (Web site) to learn about research trips into the caverns. Nearby attractions • National Corvette Museum, Bowling Green, Ky. 1-800-538-3883; www .corvettemuseum.com. Tours are also available at the adjacent Corvette assembly plant, Web site. • Nolin Lake State Park, Bee Spring, Ky. Camping, boating and fishing along a 5,795-acre lake north of Mammoth Cave National Park. Web site. • Abraham Lincoln Birthplace National Historic Site, Hodgenville, Ky. Site of Sinking Spring farm where Lincoln was born on Feb. 12, 1809. 270-358-3137, Web site.

As you read this, spring's warmth is becoming summer's often oppressive heat. What better place to retreat from the sun than the depths of the world's longest cave (more than 350 miles of tunnels have been explored and mapped) where the temperature is always a cool 54 degrees?

The cave's pleasant atmosphere is just a small part of the allure of this geological wonder folded into the limestone hills of central Kentucky. Whether you are simply curious about what may be hidden inside the passages or are an avid caver eager to search for undiscovered pathways, Mammoth Cave features treks for every interest and ability.

Like the New York City subway, Mammoth has several levels of tunnels, created over millions of years as rainwater flowed through the porous limestone on its downward journey to the Green and Nolin rivers that run through the park.

Over eons, the upper passages were left high and dry, ideal conditions for the slow growth of wondrous gypsum deposits, some hundreds of thousands of years old. Visitors marvel at these fragile formations, in all shapes and sizes, from delicate fans and flowers to ice cream scoop-size formations that fill the ceiling of the "Snowball Room" (now the cave's snack bar!).

Because water moved so rapidly through the cave, Mammoth is remarkable for the near absence of stalactites and stalagmites that are often associated with damp caves. Notable exceptions are the dramatic flowstones in a popular section appropriately called "Frozen Niagara."

Mammoth Cave's human history is as colorful as its geology. Native Americans first entered the cave nearly 4,000 years ago, traveling as far as 10 miles with only crude torches to light their way.

Surprisingly, they seemed to have ceased exploring more than 2,000 years ago. The cave was forgotten until, as legend goes, settler John Houchins rediscovered the entrance while hunting game in the 1790s.

Following the War of 1812, when slaves mined the cave's nitrate-rich soil to make gunpowder (the wooden vats may still be seen near Booth's Amphitheater), word spread of the mysterious "mammoth" cave, and intrepid tourists braved the trip to the remote frontier to see for themselves.

The cave became a bona fide tourist attraction when Franklin Gorin purchased it in 1838 and employed slave guides to lead visitors (including Ralph Waldo Emerson) into the tunnels.

One guide, Stephen Bishop, began leading trips at 17 and spent his own time exploring miles of uncharted passages. He was first to describe massive Mammoth Dome and the Snowball Room, and he published the first map of the cave in 1843. Bishop died in 1857 and is buried with other slave-era guides in a cemetery near the park's visitors center.

By the turn of the 20th century, struggling farmers turned to cave tourism to survive. Competition among a score of private caves (most were simply remote entrances to Mammoth Cave) was fierce, and disputes occasionally became violent. Eventually fed up with the poor publicity created by these "cave wars," Kentuckians urged the federal government to step in and protect the cave.

In 1926, President Calvin Coolidge signed legislation making Mammoth Cave a national park.

Unlike western parks, much of Mammoth Cave was beneath private land that had to be purchased to create the park. With funds appropriated by the state and contributions from thousands of Kentuckians, more than 50,000 acres of mostly worn-out farmland were eventually acquired.

In 1933, as the nation was in the depths of the Depression, work on the park began with the arrival of nearly 600 young men from the Civilian Conservation Corps, who built trails in the tunnels and on the surface, cleared ground for campsites, removed hundreds of abandoned and deteriorating buildings, planted millions of trees and erected dozens of park buildings, many still in use today. Work was completed in 1941, but America's entry into World War II postponed the park's dedication until 1946.

Declared a World Heritage Site in 1981, Mammoth Cave is a subterranean treasure and a green oasis. Sun-seekers enjoy hiking and biking miles of trails, fishing and boating on the Green and Nolin rivers (boat rides are offered April through October aboard the Miss Green River II) and taking leisurely drives along miles of quiet, tree-shaded lanes.

For most park visitors, though, the real fun remains underground. Guides offer 12 cave excursions, from the 90-minute Cave Discovery Tour, to the half-day, 4-mile trek along the wide Grand Avenue, to the Wild Cave Tour where budding spelunkers don helmets and get down and dirty, to the Trog Tour, a 1-mile stroll just for kids.

A large campground is close to park headquarters and a smaller, primitive campground is near Houchins Ferry, on the western side of the park. Accommodations in motel-style rooms, chalets, cabins and summer cottages are available through the Mammoth Cave Hotel, just a short walk from the visitors center.

Summer is the most popular season, so plan your activities well in advance.

If you choose to visit in winter, the cave is still a constant 54 degrees — delightfully warm down under.