What did @swhole.animal actually say?
The creator claims there are two routes to underwater vision. One involves children under 18 developing "a second layer on your eye" through prolonged underwater exposure, citing Southeast Asian sea nomads. The second is something most adults can apparently learn: deliberately blurring your eyes to change the "sphericalness" of your lens, then finding a sweet spot of partial blur that compensates for how water bends light differently than air. The pitch is essentially that you can train voluntary lens control and dial it in underwater.
It sounds compelling. It also collapses the moment you apply basic ocular anatomy to it.
Does the science back this up?
Partially, but only for the first claim about the Moken people, and even that gets the mechanism wrong. The voluntary blur-training method has no peer-reviewed support as a learnable skill for underwater clarity.
The Moken and Bajau sea nomads of Southeast Asia have been studied, most by Anna Gislen and colleagues in a 2003 paper published in Current Biology. That research confirmed Moken children could see underwater roughly twice as clearly as European children. But the mechanism was not a structural "second layer." It was pupil constriction, the smallest pupils ever recorded during underwater viewing in humans, combined with a greater-than-normal ability to accommodate, meaning the ciliary muscles could change lens curvature more aggressively than average. Gislen's follow-up work in 2006 (also in Current Biology) showed European children could be trained to similar levels of underwater acuity in about a month. Adults showed much less plasticity. No anatomical second layer was ever identified or proposed in this literature.
What did they get wrong (or right)?
The Moken claim is real in spirit but wrong in mechanism. Saying they develop "a second layer on your eye" is simply inaccurate. The actual finding involves pupil and accommodative adaptation, not a new ocular structure. That distinction matters because the creator uses the wrong mechanism to build their second claim: that you can volitionally blur and then "pull back" your lens into a water-optimized shape.
Here is the core problem. The human lens is not under direct voluntary control. You cannot consciously instruct your ciliary muscles the way you flex a bicep. What most people can do voluntarily is defocus by relaxing accommodation, which allows the lens to become more spherical through passive elastic recoil. The creator interprets this as actively controlling lens shape. It is not. You are releasing tension, not adding a new command. More importantly, the refractive mismatch between the cornea and water, which is responsible for the bulk of underwater blur, is not corrected by lens accommodation alone. The cornea accounts for roughly two-thirds of the eye's focusing power in air, and that contribution collapses in water because water and corneal tissue have nearly identical refractive indices. No amount of lens tweaking fully compensates for that loss.
What should you actually know?
The underlying neuroscience of visual adaptation is genuinely interesting, and the Gislen studies are worth reading. But the framing here overestimates what voluntary control can do and misrepresents the biology of how the Moken actually see underwater.
What the research actually supports:
- Children's visual systems are more plastic and can adapt accommodative range through practice (Gislen et al., 2003 and 2006, Current Biology).
- Pupil constriction, not a structural layer, is a key driver of improved underwater acuity in trained individuals.
- Adults retain some ability to improve underwater accommodation but show less adaptation than children.
- Wearing flat-lens dive masks solves the corneal refractive problem entirely, which is why they exist.
The idea that you can casually train yourself to see clearly underwater by practicing deliberate blurring is not supported by any study in optics, optometry, or vision science. The creator is describing a real sensory phenomenon, voluntary defocus, and extrapolating far beyond what the evidence allows. That extrapolation is the problem.