Depth charge: freediving

William Trubridge, holder of 15 freediving world records, wrote: "I have a relationship with the depths, they beckon me beyond my means, cold dark vacant pressure, forever night, endless dreams." The 32-year-old New Zealander knows intimately the literal depths of the planet's waters.

With no fins, no tanks, no weights, no help whatsoever - nothing but a swimsuit and his own body and mind - Trubridge became the first person to swim alone straight down over 120 metres into the endless ocean, with only a vertical guide rope to pull himself down to the depths and back up to the sunlight.

If such an act of human endurance sounds incredibly extreme, that's because it is.

"The deepest dives last in excess of four minutes, but that's not four minutes of holding your breath in your bathtub - it's four minutes of propelling yourself through the water column, while combating pressures that would crush a soccer ball to the size of a tennis ball and exert mind-numbing narcosis on your neural circuitry," explains Trubridge, who holds a bachelor of science degree in physiology and genetics. "It's four minutes that takes place in another dimension, where time is drawn out into an eternity - an eternity that lasts but a single breath."

Freediving, meaning diving that is free of scuba tanks or any artificial breathing apparatus, is entering the mainstream like other formerly elite activities, such as skydiving, bungee jumping or kite surfing, already have. Like all extreme sports, the very element of danger is what makes freediving so intriguing. And with proper instruction and guided practice, anyone fit enough can experience this otherworldly discipline.

The last 10 years have seen a rapidly growing number of freediving schools and organisations around the world, as well as professional freediving competitions. For beginners, a wide selection of instructor-led freediving holidays are available in places as far-flung as Egypt, The Azores and the Philippines.

Trubridge teaches half a dozen courses in the Bahamas and Italy every year. He was the first to discover Dean's Blue Hole in the Bahamas, now recognised as the world's premier freediving venue and the place where he takes holidaymakers on freediving courses. Encircled by a natural rock amphitheatre, Dean's Blue Hole is the world's largest vertical sea cavern and plunges over 40 storeys down into the liquid depths. It's also the site of the annual Vertical Blue freediving championships, now in its fifth year.

But he's not the only freediving champion organising holidays. Former world record holder Umberto Pelizzari oversees freediving courses in the Red Sea and the Mediterranean. And four-time world-record holder Fred Buyle teaches freediving instructor courses in The Azores, in addition to hosting an annual freediving holiday on a live-aboard yacht in Greece's Saronic Gulf.

"Greece is an ideal location at the end of the summer. It's peaceful, the water is warm and the locations are stunning," Buyle explains. "I think a sailboat is the best way to experience freediving. Both activities are eco-friendly and, when spending a week on a sailboat, there's no need to worry about transfers to the dive site, losing time finding a place to eat and so forth. Everything is on site and people can spend more time with instructors too."

You don't need to be half-fish to enjoy a freediving holiday. Humans are biologically suited to the sport. Having floated in fluid for the first nine months of our lives, infants instinctively hold their breath for up to 40 seconds while making swimming motions when placed in water.