Scientists in scuba gear rarely get up close and personal with marine animals, because their noisy breathing scares the creatures away.
But professional freediver Christian Redl, 39, who has plunged to depths of 95m just by holding his breath, has managed to get nose to snout with a grey reef shark, and looked a humpback whale in the eye.
These magical moments have prompted the Austrian to support a number of marine conservation projects around the world, and are what spur him to continue the sport of freediving.
"Freediving is like flying underwater. You are light, and everything is silent," said Mr Redl, who can hold his breath for seven minutes. "And with freediving, I can go almost 100m underwater. In scuba diving, the maximum is only about 30m."
He has been diving without breathing equipment for almost 20 years, pushing himself to extremes.
Last month, Mr Redl claimed a world record - his ninth - for being the first person to freedive in the frigid waters of the North Pole. He was accompanied only by polar expedition leader and professional photographer Marcus Fillinger.
Mr Redl plunged 30m under ice cover 1.8m thick, and lasted a minute and four seconds in the relatively warm -2 deg C waters. Above ground, the temperature was -30 deg C, freezing on the spot the tears of joy Mr Redl shed at fulfilling a lifelong dream.
"On the way down, it was so dark and I couldn't see anything," he said of the experience.
"But on the way up, everything was clear, I could see hundreds of metres in all directions."
In comparison, the visibility in many lakes is only about 5m, and in the famously clear waters of the Red Sea in Egypt, it is about 30m.
"There were no animals, but the green of the ice was so beautiful... I wanted to do it again."
So he did, twice more.
The "Iceman" spoke to The Sunday Times while in Singapore last Monday as brand ambassador to Swiss watchmaker Edox, which had paid for the US$80,000 (S$107,000) Arctic expedition - his latest feat in a string of other extreme freediving stunts.
In 2012, Mr Redl broke another world record when he achieved the highest freedive at the Gokyo Lakes in the Himalayas, 5,160m above sea level. There, oxygen makes up only 14 per cent of the air - a third less than at sea level.
At such altitudes, the heart rate also goes up. This is bad news for the freediver, who tries to slow his heart rate as much as possible to extend his bottom time.
"Nine out of 10 doctors told me it was impossible to go freediving at such altitudes," Mr Redl said. But against the odds, he did. It is a feat yet to be repeated.
His extreme stunts have made Mr Redl a prime test subject for dive doctors and researchers interested in learning more about the human body.
As Dr Jason Lee, head physiologist at DSO National Laboratories, put it: "The extreme depth and duration (Mr Redl) stays underwater... presents a rare opportunity for research."
"Such research efforts would allow us to explore human limits, providing valuable insights to support efforts in clinical treatment," added Dr Lee, who is Adjunct Associate Professor at the National University of Singapore's Yong Loo Lin School of Medicine.
Mr Redl's 2012 Nepal dive yielded important results for high-altitude and dive doctors.
"We found that with a good training plan, anyone can go 5,000m up without altitude sickness. The problem is that many climbers don't train before they go," he said. "What doctors did is that they created a training table and breathing exercises for mountaineers... and I believe this could save lives."
Doctors found that Mr Redl's lung capacity shrank by two-thirds due to the pressure when he dived under ice.
When a freediver goes underwater, a natural response called the mammalian dive reflex kicks in. The heart rate slows down and blood vessels constrict, causing increased pressure on the arteries. These combined reflexes conserve oxygen and redirect blood to the brain and heart, to prevent key organs from collapsing.
It is likely that all mammals have this capability to help them stay underwater for extended periods, said Mr Marcus Chua, museum officer and curator of mammals and birds at the Lee Kong Chian Natural History Museum.
"For non-aquatic mammals like humans, the reflex may help preserve their lives if they fall into water," he explained.
One way to study it is to have someone immerse his face in a basin of water, said Associate Professor Soh Chai Rick, director of the Hyperbaric and Diving Medicine Centre at the Singapore General Hospital (SGH).
"This allows equipment to be attached without requiring him to be submerged," said Prof Soh, who is also a senior consultant at SGH's Department of Anaesthesiology.
But data collected on the surface is different from information collected underwater, especially when it comes to deep dives. The pressure, for instance, affects the time a person can hold his breath.
So Mr Redl wears a dive computer on his wrist to track his heart rate, and has blood drawn underwater for testing later.
He also volunteers at a shark research institution in the Bahamas.
There, he teaches students how to interact with sharks and not to fear them. He is usually accompanied by a team of doctors and safety divers when he attempts to break a world record. His entourage, which can number up to 10, not only ensures his safety underwater but also helps to collect data on his heart rate and lung capacity.
His stint at the North Pole, however, was done without the usual team because of cost and the difficulty in putting together a group who could endure the cold.
Mr Redl himself took about a year to train for the expedition - twice the time that he usually gives himself for record-breaking feats. His regimen included cardio exercises in the gym, diving in the icy waters of Lake Weissensee in the Austrian Alps, and spending a week in the Arctic to get used to the cold.
His latest adventure is more than just another notch in his belt.
"I also wanted to raise awareness about what climate change is doing to the North Pole - the temperature is getting warmer, the ice is getting thinner and, recently, there has been no ice in the summer," he said. "This is a big problem for the polar bears there... they have no space to live."
This article was first published on May 24, 2015.
Get a copy of The Straits Times or go to straitstimes.com for more stories.