A subterranean adventure awaits in the Bluegrass State. Kentucky is home to Mammoth Cave National Park, the most extensive cave network in the world -- 400 miles have been explored and many more have not. Descend into its depths on a variety of guided tours starting at $7 for adults and $5 for children, with more challenging excursions at higher prices. (Try the free-climbing, lengthy crawls, wet areas, and tight openings of the "Wild Cave Tour" for $55, ages 16 and up only.)
Get up close and personal with the raw force of one of nature's most beautiful wonders at Hawaii Volcanoes National Park. It contains two of the grandest and most active volcanoes on record: Mauna Loa, the largest active volcano in the world, and Kīlauea, known more for its aggression and activity, spewing rivers of molten lava every day. Passes for $25 a vehicle are good for seven days.
A few hours from Chicago and Minneapolis is a natural wonder reminiscent of Pacific Coast rock formations -- in Wisconsin. Tour the sea caves of the Apostle Islands by kayak, paddling through tunnels and watching the sunset from the water under sandstone archways after emerging from hidden wraparound caves.
Cascading involves climbing up creeks to rappel or jump from amazing waterfalls. Natural landscapes, roaring water, dizzying heights, and death-defying jumps are all part of the package. A Ciguapa Falls tour includes 12 jumps of up to 30 feet and total river time of about 2.5 hours. (Expect some steep climbs, and know how to swim.)
Price: $99 to $129
Few activities are more exhilarating than racing through the air at 30 mph, far above one of the most beautiful mountain landscapes in the world. Off the coast of California, adventurers travel nearly three-quarters of a mile on five zip lines 600 feet above sea level. Each stop includes an expert presentation about the geography, wildlife, landscape, and history of Catalina Island.
Price: $129 a day
Horseback riding is an adventure in and of itself, but the stakes rise as part of an active roundup on a working cattle ranch. The Two Creek Ranch in Wyoming, spanning more than 25,000 acres on the North Platte River -- next to the legendary Oregon Trail -- offers a full week of sorting, branding, calving, gathering, trailing, and shipping cattle.
Get lost in the rugged beauty of Arizona with a half-day mountain bike excursion that pedals through the majestic Sedona Mountains on some of the best-connected and most extensive bike trails in the country. Tours are private -- and geared for every body type and skill level -- but the bigger the group, the lower the price for each person.
Real-life sea monsters, sharks are the ocean's apex predators -- and the brave can swim alongside them in waters off the Florida Keys. No scuba certification is needed, and all equipment comes with the package, including the cage that separates divers from prowling bull sharks, tiger sharks, mako sharks, reef sharks, and alien-esque hammerheads.
Price: $245 to $265
What would a list of adventure trips be without skydiving? In Key West, Florida, skydivers reach speeds in excess of 120 mph, then float slowly to Earth once the chute is pulled. They may spot dolphins and tropical birds below while steering to a pinpoint landing. Packages come with instruction and equipment.
Price: $250 and up
Operators compare the experience of soaring above scenic Lake Tahoe to "riding a motorcycle in the sky." After taking lessons in operating a powered hang glider and being in the pilot's seat in full control, participants come home with a license to fly an Evolution Revo trike, a sport aircraft registered with the Federal Aviation Administration.