Is white-water rafting no longer a rush? Been there, done that with bungee jumping? If standard travel activities like hiking, surfing, diving and zip-lining are leaving you cold, check out these unique, crazy, and sometimes downright dangerous adventure sports around the world.

Zorbing

If the idea of rolling downhill in what is essentially a human hamster wheel appeals to you, you may be interested in zorbing. Zorbing (or sphereing) was developed in New Zealand, though there are now several countries that offer facilities for this unusual experience.

All that's really needed is a gently sloping hillside (or man-made ramp) and the zorb itself - a ball made of flexible plastic with a smaller round chamber inside. Water is often added to help riders stay stationary as the ball rolls around them. Zorbers climb in and the ball is pushed down the rill, rolling for about a half mile until it comes to rest and the rider climbs out, exhilarated, disoriented and often more than a little dizzy.

Diving with Great White Sharks

Most people spend their time in the water trying to avoid sharks, and while adventurous divers are often thrilled to encounter generally non-hostile varieties like reef sharks, most won't want to come face to face with a Great White. But in coastal areas of the Americas, Australia and South Africa, you can do just that.

Don your SCUBA gear, climb into a cage, descend below the surface, and wait. Though there's no guarantee you'll see a Great White, most companies do report high success rates for sightings. Struggle to control your racing heart (studies show Great Whites react to faster heartbeats) as the sharks circle closely around the cage, and try not to contemplate the damage these 20-feet, 4,000 pound predators can do with a single bite.

Kitesurfing

Kitesurfing (also called kiteboarding) is pretty much just what it sounds like - surfing with a kite. Surfers who've mastered the art of riding waves can perform bigger jumps and twist with the aid of a kite that catches the wind. Similar to wakeboarding, but without a boat, kitesurfing allows surfers to go out father and rely less on the strength of the waves for their maneuvers.

Kitesurfing can be done at any beach where there is a strong and reliable wind, though many public beach areas have banned the sport due to safety and traffic concerns.

Bridge Climbing

The urban version of mountain climbing, bridge climbing isn't the most strenuous activity but it can still be an exciting challenge, especially if you have a fear of heights. There are only a few places in the world where you can (legally) scale a bridge. The Sydney Harbour Bridge is one of them.

The climb takes about 3.5 hours, and travels along the upper span of the Bridge's arch. Climbers maneuver along catwalks and up stairs and ladders all the way to the summit, where they'll be rewarded with unobstructed views of the city below.

Free-climbing

Free-climbers flaunt the law of gravity as they climb rocks and mountains using only their hands and feet to climb without the aid of ropes, pulleys or picks. Most free-climbers still use safety ropes as a precaution, but some daredevils go without (known as free-soloing).

Free-soloers often stick to well-known climbs within their climbing ability, but the sport still carries a lot of risk. For experienced climbers bored with their usual routes though, the dangers of free-climbing and free-soloing may just add to the adrenaline rush and challenge of the climb.

Sand Surfing

Necessity is the mother of invention, a fact no sport demonstrates better than sand surfing. Since waves for surfing can be as hard to come by in the desert as pavement for skateboarding, some savvy desert-dwellers conceived the sport of sand surfing (also called sand boarding) - which combines the concepts of both sports and adjusts them to the specific constraints of working on sand.

In countries like Namibia, South Africa, and Peru, surfers climb to the top of sand dunes, strap on a board similar to a snow board and criss-cross their way down the sandy slopes, turning and jumping as they go.

Cold-Water Swimming

Known by many as a Polar Bear Swim, the winter act of swimming in frigid water isn't so much an adventure sport as it is a test of endurance. . . or insanity. Despite the fact that just a few minutes in freezing water at the height of winter can result in hypothermia, hardy souls from Chicago, Illinois to Harbin, China still take the icy plunge.

If you fancy a cold dip, you can join them. Strip off your clothes, dash over the snow and jump in the freezing water to see how long you can last before you emerge blue and shivering. You won't do much actual swimming during a Polar Bear Swim, but you many discover a newfound appreciation for the warm waters of your own bathtub.

Heli-Skiiing

To reach the highest peaks, extreme skiers don't take a chair lift. They rely on a slightly more expensive form of transportation - a helicopter. Heli-skiing is the sport of skiing (or snowboarding) in which skiers are lifted to the tops of mountains by helicopter and then ski downhill, far off the beaten paths used by resort skiers.

Available in mountainous areas all over the world, heli-skiing terrain varies from glaciers to wooded areas to steep, jagged chutes. The risk of falls and avalanches makes heli-skiing a risky activity, but it offers triple-black-diamond level experienced skiers the chance to explore terrain not found on the average resort run.

Photo credits: Shark diving by valkyrieh116 on Flickr, Zorbing by Jonathan_W on Flickr, Kitesurfing bytechnosurf on Flickr, Bridge climbing by Niels' on Flickr, Free climbing by .sanden. on Flickr, Sand surfing bysixintheworld on Flickr, Cold water swimming by Margarita Banting on Flickr, Heliskiing by DennyMont on Flickr

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