This weekend, the U.S. National Park Service (NPS)  is offering free admission to all of the parks, monuments, recreation areas, and other units that comprise its impressive, 84 million-acre system. So, pack up the car and head to the park nearest you.
If you’d rather not battle the summertime crowds at Yosemite  or the Grand Canyon , remember that the NPS protects 391 units throughout the United States. To escape the heat, I prefer delving into an underground cave, where the temperature is constantly cool. Luckily, the NPS preserves several intriguing cave systems, including Wind Cave National Park  and Jewel Cave National Monument  in South Dakota, Oregon Caves National Monument  in Oregon, Timpanogos Cave National Monument  in Utah, and Russell Cave National Monument  in Alabama.
While I recommend visiting all of the above, no one should miss Carlsbad Caverns National Park  (575/785-2232) in New Mexico and Mammoth Cave National Park  (270/758-2180) in Kentucky – two places that I would love to explore on my own, armed with a generous water supply, several light sources, and a keen sense of adventure. Of course, knowing me, I’d get lost within the first hour.
Most visitors to Carlsbad Caverns head first to the wondrous Big Room, where a self-guided trail winds amid an assortment of cave formations, from the Painted Grotto to the Bottomless Pit. To venture beyond this well-trodden place, reserve  a spot on one of the park’s ranger-led excursions, including the mazelike Spider Cave Tour and the four-hour Hall of the White Giant Tour.
From mid-April to mid-October, I urge you to stay for the Bat Flight program near dusk. Weather permitting, curious visitors can head to the outdoor amphitheater and relish the remarkable exodus of Mexican free-tail bats as they flee the caves for their evening insect hunt. Watching thousands upon thousands of bats rushing from the darkness and swirling into the sunset is an unforgettable experience. Just remember: No cameras are allowed during the bat flight, so the only pictures you can take with you are mental ones.
By far, my favorite cave system in America is Mammoth Cave. With over 365 miles of twisting passageways and airy chambers, it’s unbelievably the largest known cave system in the world. To experience it properly, my husband, Dan, and I took the three-hour Introduction to Caving Tour.
Clad in jeans, boots, gloves, kneepads, and helmets, we followed the tour group into the chilly caves. Past a few well-lit caverns, we were instructed to squeeze through a keyhole in the rocks. When all but two ladies (who opted for an easier tour) had surfaced, we turned on our headlamps and edged toward our first climb.
For the next couple of hours, we passed walls of fragile gypsum, tiptoed along narrow canyon ledges, and crawled through spaces no more than a foot high. We felt like trailblazers of a subterranean wilderness, especially when we all turned off our headlamps and contemplated how dark the caves must have seemed to early explorers.
Afterward, we emerged into the forest and surveyed the damage: dirt as far as the eye could see, the mark of a grand spelunking adventure. Grateful to have seen a different side of the world – hidden by most surface-dwellers – we vowed to return someday for the six-hour Wild Cave Tour, an intense underground trek that claustrophobes might want to avoid.
Regardless of where you go, there are some caving guidelines to remember. To preserve the caves, avoid touching fragile formations and refrain from leaving any trash behind. To protect yourself, always travel with others, tell at least two people where you’re going, bring at least three different light sources with you, and, if you ever get lost, stay in one place so that your group can find you. For more information about cave conservation and safety, consult the National Speleological Society .
With that, I wish you well on your journey this weekend – and hope that, no matter where you go, you enjoy America’s parks as much as I do.