There's a hierarchy in the world of extreme vacations.
Sky-divers start at 10,000 feet in the air, plummeting from a plane with a parachute. A little closer to earth, long on courage and short on sense, BASE-jumpers hurl themselves from cliffs to sate their adrenaline desire.
These are mere pastimes. The real action begins below sea level: swimming with sharks.
For most people, the word "shark" prompts a knee-jerk fear fed by blockbuster films and a general feeling of insecurity in the ocean depths.
But many others get baited by the thrill of swimming among these underwater predators, away from artificial aquariums and the safety of the movie screen.
"You get a distinct feeling of where you are in the universe," chuckles Patric Douglas, CEO of SharkDiver.com, a shark-diving adventure company. "You realize you're not top of the food chain."
Traditionally, those brave enough to enter the chum-filled water cocooned themselves in reinforced cages, or stood aboard the ship and ogled as ravenous sharks were fed from the deck.
But for some that's just not a strong enough buzz. Searching for new extremes, SharkLady Adventures, a company in South Africa, developed a Plexiglas tube to give the diver the real sensation of swimming with sharks.
"[The diver sees] the great white in its natural environment with crystal clear clarity vision," says Kim "Shark Lady" Maclean, head of the company. "[It is] a one-on-one encounter with the magnificent great white."
Still, it seems that for some, the Plexiglass offered too much protection.
The solution? Shark riding.
Ocean explorer Jean-Michel Cousteau and shark tour guide Andre Hartmann pioneered this new interactive experience off the coast of South Africa.
As portrayed in Cousteau's film, "Sharks at Risk," the intention of the expedition was to disprove the popular myth that sharks are bloodthirsty killers.
The footage depicts the two divers cautiously approaching a great white shark. Carefully positioning themselves near the animal, they take turns grabbing the shark's dorsal fin to be whisked away before safely letting go.
But while many shark-diving companies, including Cousteau's, say they want to protect the animals. Scientists and industry members warn of the impact on sharks and their habitat.
"The animals are going to be affected, affected in the sense that their behavior is going to be changed," says Robert Hueter, director of Mote's Center for Shark Research in Sarasota, Fla.
Hueter also warns of the potential danger shark diving poses to the health of the animals. "One thing it can do, it can bring the sharks together in densities that are not normal and it does provide the potential for them to spread pathogens and infections."
But Hueter insists he is not opposed to shark diving per se, "[as long as] they're well controlled and concerned for the animals ... it should help fuel conservation."
David Campbell, director and founder of MarineBio.org, argues the real dangers to sharks lie elsewhere, that fishing does more damage than by the shark-diving industry.
Others fear the growth and exploration of free diving with sharks.
"We're there at their discretion and should stay in the cages," warns Douglas. "[Shark diving] is OK as long as there's a barrier between you and the animal.
"If you're with a top sea predator, be a passive observer. You're a guest in their home. You can't look at it like a biological ATM."
Looks like those hunting an adrenaline nirvana will have to stay in their cages or look elsewhere.