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I had never heard of this happening before nor can I find anything online but this happened to me years ago, did not try it again and I wonder if once you realize the illusion it no longer works.

As I recall, I was standing on the shore and I think simply the water suddenly receded very rapidly while my feet were at the edge. I immediately felt a sensation of very fast motion.

I wish (when I lived near the beach) I had experimented more, seeing if indeed I would feel this each time, despite now expecting it to happen. I also did not ask my friend to try it.

Is this a known phenomenon? If so, what is it called? And will everyone experience this effect, including people who have experienced it before and expect it to happen to them again?

As I write this, it seems unlikely that I in fact did not try this again. I am now wondering if the nature of the wave, perhaps due to chance, time of day or something about that specific beach (or some combination of factors) caused this illusion. One aspect of this is that the illusion was so pronounced that I think I felt like I was losing my balance -- this happened most of a lifetime ago and I am a little hazy on the details.

One final factor may have been exact timing -- perhaps I was looking not at my feet but into the distance and by chance I shifted my gaze to the ground at the right moment.

I suspect there are many situations where such illusions of movement occur.

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Self-motion can be perceived based on optic flow - it seems most likely that you experienced an illusion based on this. If you are familiar with the "Star Trek" series at all, think about how the rapid movement of a spacecraft is depicted as stars flowing backward in the opposite direction to the ship (certainly the same trope is used in other media as well). That's a good example of optic flow: it's the stars that move on the screen, but you perceive the ship as moving. Stars are convenient as an illustration, but any visual object can cause optic flow.

If you think about perception as a process of predicting the causes of the things around you, if the "whole room" seems to move in one direction, the simplest explanation might be that actually you are moving. In this case, your perception briefly got it wrong, and that's the essence of illusions. The sense of loss of balance occurs because of conflicting information between your vestibular system and vision.

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