How can you dive 700 feet in one breath?

People started Ancient Greek divers weighted themselves down and descended to depths of 100 feet (30.5 meters) or more to pluck sponges from the sea bottom. In Japan and Korea, traditional female freedivers known as Ama have long collected edible seaweed and clams at depths of 65 feet (19.8 meters) or more [source: Hanna]. Because coastal dwellers have been freediving for many generations, you might suspect that they've developed special genetic traits that make them better at it than inlanders. But scientists who've studied them say there's no evidence of that [sources: Focazio].

All humans do share an evolutionary adaptation with air-breathing aquatic animals that makes it possible for them to dive and swim underwater for a while without taking a breath. In 1870, French physiologist Paul Bert observed that ducks had an amazing ability to slow their heart rate if they were forced to stay underwater. Scientists went on to discover that immersion in cold water -- or even simply holding your breath -- triggers something called the diving reflex, which the body uses to conserve life-giving oxygen. When the reflex kicks in, your pulse slows and your circulatory system constricts, and blood is shunted away from your muscles, skin and visceral organs, guaranteeing that your heart and brain get first dibs [source: Bove].

For years, scientists believed that humans were capable of enduring without oxygen for only a few minutes and that they couldn't dive deeper than 164 feet (50 meters), because the pressure would shrink their lungs and crush their rib cages. But in the 1960s and 1970s, divers disproved that myth. As it turns out, a phenomenon called blood shift causes the lungs to fill with plasma, preventing collapse [source: AIDA New Zealand]. Learn how freedivers put blood shift to work on the next page.

In 1976, French diver Jacques Mayol became the first human to descend to (328 feet) 100 meters [source: yoga and the breathing techniques of kung fu-practicing Shaolin monks, adapted those mind-body control methods to slow his pulse from 60 to 27 beats per minute underwater, enabling him to dive deeper and longer. In doing so, he revolutionized freediving and paved the way for Herbert Nitsch's generation of divers to push the limits even further.

It's fairly easy to get into freediving. It doesn't require a lot of expensive equipment -- a wet suit for warmth, a mask and a set of special flippers designed for underwater efficiency can be had for less than $1,000. And according to freediving expert and author Terry Maas, with competent instruction and a little practice, novices quickly can learn to stay under for 45 seconds, long enough to descend as far as 30 feet (9.1 meters) and experience the ocean from a startlingly new angle [source: Maas].

But going without oxygen for longer periods and diving to serious depths isn't for dabblers. Elite divers must endure rigorous training to develop their lung capacity and control their pulse rates. They also utilize special safety equipment, such as balloon systems to help them return to the surface more quickly.

Even then, tragedies sometimes occur. In 2002, elite French diver Audrey Mestre was attempting to set a no-limits depth record off the coast of the Dominican Republic, when equipment malfunctions apparently kept her underwater too long. She lost consciousness during her ascent and perished [source: IAFD]. Another French champion freediver, Loic Leferme, died off the south of France in 2007 when his ascent rope became snagged [source: Dive Magazine].

Learn more about water sports and other adventurous activities by clicking through the links on the next page.