Many people think of free diving - a sport in which enthusiasts descend deep into the sea on a single breath - as a high-risk pursuit. And it often is, especially when divers are pushing their physical limits. Divers may pass out and drown, or go so far below they can't return to the surface in time.
A small group of adventure-seekers in Hong Kong have become hooked on the sport, although they are mostly drawn more by the ability to get close to marine life than by the brush with danger.
Simon Lau Sai-man is among the first Hongkongers to take it up. The 39-year-old says he has loved water sports since he was little and turned to free diving in his teens because he couldn't afford scuba gear. Lau has since become a full-time scuba instructor, but he says there's nothing quite like free diving.
He recalls a dive in Puerto Galera, in the Philippines, when he descended to a depth of more than 50 metres and spent several minutes swimming alongside dolphins and whale sharks, so close he had to fight the urge to touch them.
"I felt as if I had become one of the marine creatures," he says.
The world's earliest free divers were probably spear fishermen and pearl collectors; historians believe the people of ancient Mesopotamia dived for pearls in 4,500BC. But free diving began to emerge as an extreme sport in the 1960s, when Europe became enthralled by the rivalry between two men - Enzo Maiorca, a Sicilian who was the first to dive below 50 metres, and Frenchman Jacques Mayol, who first beat him by 10 metres and then dived to 105 metres. When in 1988 French film director Luc Besson made The Big Blue, a fictionalised tale of the two champions, interest in free diving grew around the world.
Because scuba divers are breathing compressed air, they release streams of bubbles as they exhale, which often scares fish away. Free divers can get much closer, since they are unencumbered by breathing apparatus, says Jonn Benedict Lu, 38, a Singaporean filmmaker now based in Hong Kong.
Lu, who has been free diving for nearly a decade, describes a dive in the Philippines this year, when he met a pair of sea turtles.
"I think they were curious about me. They looked and circled around me before moving on. Rather than being an intruder or an alien in the ocean, I felt very much part of it."
Lau says: "I visit fishes in the sea at least once a week - I don't even see my best friends as often."
From just five people taking up free diving in Hong Kong a decade ago, Lau has seen the number of enthusiasts rise. That's due partly to his classes: he has taught the basics to about 200 people, although only about 50 dive regularly.
"People tend to think scuba diving is the only way to go under water - it will take time before they realise that free diving can be a fun, dynamic and environmentally friendly alternative," he says.
For computer scientist Alexander Markowetz, free diving is like doing meditation under water.
"Whenever I feel irritated or upset, I go free diving and always came back refreshed and energised. I feel so carefree surrounded by the womb-like ocean," says the Belgian, who spent five years as a research scholar at the University of Science and Technology before relocating to Germany in January.
"It's much easier and cheaper than scuba diving, and it allows more flexibility under water. I feel like a dolphin doing free diving and a whale wearing scuba equipment."
In Hong Kong, Markowetz used to free dive several times a week, donning a wetsuit and plunging in.
"It's so quiet under the water. I don't know of many places in Hong Kong where you won't hear noise - people chatting or mobile phones going off," he says.
That's probably how people such as Lu get hooked on the sport.
"I get a very odd sensation," says Lu, "I feel like I don't want to come up to the surface, I just want to stay down there for ever. It's hard to explain, but it really is addictive. "
Still, many get a charge from testing personal limits.
"It's one of the most difficult underwater sports I've ever done," says dive instructor Alan Lam Yuk-tsuen. The 24-year-old enjoys scuba diving, surfing and kite-boarding, but says free diving is his favourite.
"It challenges the diver's mental and physical condition as well as his diving skills. You need to know what to expect and be ready for it," he says.
Lau says the potential hazards of all diving can be greatly reduced if people are properly trained and take the correct precautions.
"One must be physically trained and psychologically balanced," he says.
Aspiring free divers should undergo rigorous training before taking the plunge. Classes are first conducted in pools to help students to achieve apnea - holding one's breath - with participants required to swim increasing lengths on a single breath.
If a diver can swim 50 metres on one breath in a pool, they can expect to be able dive 25 metres under the waves and return to the surface before running out of breath.
Divers are also encouraged to take up yoga to help improve control of their breathing, focus and overall performance. The idea is to lower their heart rate and slow other body functions as much as possible, so that little oxygen can go further.
"You want to fully relax yourself to accomplish apnea. Think of it as doing yoga under water," says Markowetz.
Lau, a former director of the Hong Kong Underwater Association, says staying calm is crucial.
"If you are afraid of the ocean, your level of fear will rise as you go deeper. This causes your heart rate to rise and increases risks."
Mistakes can be fatal even for top divers. In 2002, Audrey Mestre, a 28-year-old Frenchwoman, died while trying to break a record 162 metres set by her husband Francesco Ferreras. It meant descending on a device called a sled before being pulled to the surface by a balloon. But Mestre's air bag failed to inflate properly.
Lu isn't about to attempt any records. "I am not competing with anyone else but just pushing my own limits," he says.
But unexpected hazards can threaten even careful divers.
Markowetz once almost passed out from a lack of oxygen when he became tangled in discarded fishing nets near the seabed off Sai Kung. Luckily he cut himself free.
"I came up and nearly shat in my pants. I got scared, so I stopped diving for the day," he says.
Lu urges caution.
"It's a sport in which you really need to know where your limits are," he says. "If you don't, you'll black out under water and drown."
