By James M. Clash

updated 12:30 p.m. ET July 6, 2006

The top of the world, the edge of space, the 150-mph barrier in an automobile--all were virgin territory less than a century ago. Today, they are the destinations described in glossy brochures that tout extreme vacations and high-priced adventure treks. Chalk up the popularity of these adrenaline-pumping excursions to a collective midlife crisis of baby-boomers armed with enough money and technology to allow them to go where only Sir Edmund Hillary, Mario Andretti and John Glenn could go before.

What's the appeal? Ernest Hemingway once said, "Auto racing, bullfighting and mountain climbing are the only real sports...All others are games." The great American writer and adventurer had a point. He reckoned that the element of risk is an integral part of the sporting life and perhaps the very essence of the experience. Indeed, if you double-fault on the tennis court, you lose a point. Land in a water hazard on the golf course and the penalty is a couple strokes.

But strapped into an open-wheel, open-cockpit Indy race car or climbing a 13,000-foot mountain, well, in those places, your life is on the line. Misjudge the exit of a corner at 150 mph and the race driver careens into a concrete retention wall, at best destroying an expensive piece of intricate machinery and at worst snuffing out a life. Or consider the consequences of losing your footing while climbing on a knife-edged peak where a single misstep can result in paying the ultimate price for adventure.

That said, you can't eliminate risk entirely from adventure, nor would you want to. As Hemingway understood, risk is a big part of the challenge. But you can be smart about it. Correctly belayed, you're a lot safer traversing a ledge on the Grand Teton and swinging over a 1,500-foot chasm than you are free climbing only 30 feet off the ground. And a 65-mile-per-hour crash in a passenger car on the freeway will likely do more damage to the human body than a wreck on a racetrack at twice--or even three times--that speed in a specially built car with the driver wearing a snug helmet and strapped in with a proper five-point seat harness.

For instance, speed freaks can head out to the Mario Andretti Racing School. Offered at a half-dozen tracks in the U.S., the school uses a "lead-follow" program where students drive behind instructors by a few car lengths, picking up speed each lap as their comfort levels increase. The open-wheel cars and 500 horsepower are as close to Indy-style racers as you can get without driving the real things.

Seven Extreme Summer Adventures Click here to see slide show Need For Speed: Indy Cars More Need For Speed: Formula One The North Pole Mysterious Land Of Tuva The Grand Teton Edge Of Space Take A Leap Source: Forbes.com

If that's not fast enough, LRS Formula USA allows drivers to get behind the wheel of a real F-1 car. The school warms up students in smaller Formula 2000 cars in morning sessions in order to teach breaking and turn-in points on the 1.8-mile outside road course at the Las Vegas Motor Speedway. But the real fun begins in the afternoon when you are strapped into the open cockpit of a 700-hp Formula One machine that competed on the world circuit in the late 1990s. Be careful: These things accelerate from zero-to-60 mph in around three seconds. Also be careful with your wallet: Each extra lap will cost you $500 (I know, I ran five!). Recently, a Japanese student requested 40 extra laps and promptly wrote a check for $20,000.

Faster still? Space Adventures offers a MiG-25 Foxbat ride from Moscow to the edge of space, 85,000 feet above the earth. Not only do you break the sound barrier (Mach 1), you more than double it (Mach 2.5).

Forbes rounded up seven great commercial adventures appropriate for the coming summer months, with prices ranging from a few thousand dollars to several thousand dollars. All have one thing in common: They involve some sort of risk and/or hardship. In compiling our list, we sought out diversity as well as personal experience (I have done each of these offerings myself). Race-car driving, polar exploration, rare trekking, mountain-climbing, supersonic flight and parachuting made our list. (Unfortunately bullfighting did not, as I have never done it. But be on the lookout in a future story.) Enjoy!

Jim Clash's Adventurer column appears in Forbes magazine and online at www.forbes.com/adventurer. He is also the author of To the Limits (John Wiley & Sons, 2003).

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