The Moken people of western Thailand are sometimes called the Sea Nomads because they live almost their entire lives in the shallow waters of the Andaman Sea. From a very young age, they can swim as well as walk and hunt and fish in the water for their food.
They see remarkably well underwater. How? In 1999, Anna Gislén of the University of Lund in Sweden decided to find out.
BBC Future explains the problem. When we put our eyes underwater, our vision necessarily gets blurry:
Light is refracted when it enters the human eye because the outer cornea contains water, which makes it slightly denser than the air outside the eye. An internal lens refracts the light even further.
When the eye is immersed in water, which has about the same density as the cornea, we lose the refractive power of the cornea, which is why the image becomes severely blurred.
But the Moken children are taught to narrow their pupils to compensate for this change:
“Normally when you go underwater, everything is so blurry that the eye doesn’t even try to accommodate, it’s not a normal reflex,” says Gislen. “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.”
It's an impressive adaptation. But it isn't permanent. Gislén found that the Moken lost this ability as they reached adulthood.