Thousands of bats swarm out of the cave mouth each evening at sunset in Carlsbad Caverns National Park in New Mexico to hunt for bugs for food. (If you're an early riser, they return to the caves in the pre-dawn hours). The best flights to see usually occur in July and August.
National Park Service/AP/File
Glenwood Caverns, Colorado —
Glenwood Caverns co-owner Steve Beckley crawls through a crack while emerging from Beginner's Luck, a passage deep within Iron Mountain near Glenwood Springs, Colorado.
Jim Noelker/Glenwood Springs Post Independent/AP/File
Luray Caverns, Virginia —
Stalactites, at top, reflect on underground pond water at Luray Caverns in Luray, Virginia.
KAREN BLEIER/AFP/GettyImages/File
Howe Caverns, New York —
It might get hot above ground in upstate New York during the summer, but Howe Caverns at Howes Caves will be noticeably cooler underground.
Jim McKnight/AP
Niagara Cave, Minnesota —
Niagara Cave near Harmony, Minnesota, has a 60-foot waterfall and an underground wedding chapel.
Andrew Link/Winona Daily News/AP/File
Oregon Caves National Monument —
The Oregon Caves National Monument is one of the few known marble caverns, featuring rooms such as Paradise Lost, shown here.
Courtesy of National Park Service
Marengo Cave, Indiana —
Marengo Cave visitors have tossed coins toward Penny Ceiling, where they get stuck in its soft, claylike surface. Every few years, staffers remove them, shown here.
Michael Clevenger/Courier-Journal/AP/File
Penn's Cave, Pennsylvania —
For explorers who don't want to hike, consider Penn's Cave in Centre Hall, Pennsylvania, where boats carry tourists on an underground stream.
