If you want proof of how adaptable the human body can be, look no further than the Moken people - a nomadic seafaring tribe that lives in the island archipelagos on the Andaman Sea, and along the west coast of Thailand. 

Also known as sea-nomads, the Moken were once entirely dependent the ocean, and the children spent much of their time diving for food on the seafloor. Experiments have shown that they could see underwater with total clarity - a unique adaptation that other children can learn within weeks.

"Normally when you go underwater, everything is so blurry that the eye doesn't even try to accommodate, it's not a normal reflex," vision researcher Anna Gislen from the University of Lund in Sweden told the BBC. "But the Moken children are able to do both - they can make their pupils smaller and change their lens shape. Seals and dolphins have a similar adaptation."

Gislen has been working with the Moken for almost two decades to better understand this incredible ability - which only the children appear to possess - and within just one month, managed to teach a group of European children to see underwater with the same clarity.

"It was different for each child, but at some point their vision would just suddenly improve," she says. "I asked them whether they were doing anything different and they said, 'No, I can just see better now.'"

Gislen first had the idea to study the Moken children back in 1999, when she watched them dive into the ocean whenever the tide came in, and collect clams and sea cucumbers from the sea floor with their eyes wide open. 

In 2003, she published a study that measured exactly how clear their vision was, comparing 17 Moken children (10 girls and 7 boys, aged 7-13 years) with 18 European children (14 girls and 4 boys, aged 7-13 years).

The experiment involved getting the children to dive underwater and place their forehead on a headrest positioned 50 cm away from a card displaying random patterns with lines running either vertically or horizontally. After looking at the card, they'd swim back up and report which pattern they'd seen. 

Each time they dived down, the space between the lines would get thinner and thinner, making it harder to make out the pattern. The children had to repeat the process five times for each pattern, and just one mistake was interpreted as an inability to see the pattern properly.

Publishing the results in Current Biology, Gislen found that the Moken children had more than twice the visual acuity of the European children underwater.

Why? It's still not entirely clear, but she thinks it's got to do with the Moken children's ability to constrict their pupils underwater to increase the depth of field and make things clear. 

This appears to be partly solving the problem that the rest of us experience when we try to see things underwater - water has the same density as our corneas, and this messes with their ability to focus, making everything appear blurry.

The paper explains:

"On land, pupil size normally has little effect on resolution, and both groups of children were found to have the same pupil size. But underwater, when the image is severely blurred, a smaller pupil can significantly improve resolution.

Our measurements clearly show that there is indeed a difference underwater; when diving, the Moken children constrict their pupils, whereas European children do not, and pupil size differs significantly."

But as Helen Thomson reports for the BBC, this on its own isn't enough to explain the ability - they appear to also be 'accommodating' their lenses, which means consciously or subconsciously changing the shape of the lens to clarify or focus on an image as its distance varies.

Using a mathematical model, Gislen and her team figured out how much their lenses were accommodating to allow them to see as far underwater as they could, and concluded that the Moken children were able to make their pupils smaller and change their lens shape to achieve superior underwater vision.

Interestingly, in the adults Gislen tested, none of them possessed the same ability, and would hunt for food by spear fishing instead of diving. She suspects that the Moken lose their 'dolphin eyes' when they grow older because the lens becomes less flexible with age.

That's bad news for us grown-ups who dream of seeing underwater with perfect clarity, but here's the good news: as long as you're still young, it appears that you can develop the skill with a bit of practice.

Gislen recruited a group of European children on holiday in Thailand (3 girls and 2 boys, aged 9-11) and a group of children in Sweden (15 girls and 5 boys, aged 9-10) to take part in training sessions over several weeks. Basically, they had to perform similar diving tasks in the original experiment to see if repeating it would show improvements in underwater vision.

Reporting in the journal Vision Research back in 2006, she found that after 11 sessions across one month, both groups had achieved the same underwater acuity as the Moken children - even if they still got red, irritated eyes because of the salt.

The European kids ended up retaining the ability to constrict their pupils and achieve accommodation, even after four months of no underwater activities at all. "When tested 8 months after the last training session in an outdoor pool in bright sunlight - comparable to light environments in South-East Asia - the children had attained the same underwater acuity as the sea-gypsy children," Gislen and her team report.

Unfortunately, it's looking more and more unlikely that Gislen will be able to continue her research into the future. She explained to the BBC that the Moken have changed much of their lifestyle since she first visited back in 1999 - they're now spending much less time in the water.

Gislen says that while some of the children she originally studied have retained the ability into adolescence, she doubts that any newly born children will develop it. "They just don't spend as much time in the sea anymore," she says, "so I doubt that any of the children that grow up these days in the tribe have this extraordinary vision."