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It's a stereogram, you have to cross your eyes (or relax to infinity, I could never get that right) to see it. In the one linked, it shows 'John Smith is responsible' pretty clearly. Like the Magic Eye books... but in text. Works quite well actually.

https://en.wikipedia.org/wiki/Stereoscopy

Compared to some single image stereograms, it's pretty easy for me to see the 3D effect either way. The "straight" view shows the message in front of the rest, cross-eyed it's behind.

Of course, it's not particularly "secure", there would have to be layers of encryption of some kind if the goal is keeping the message hidden.

"John Smith is responsible"

Cross (or relax and sufficiently uncross) your eyes until the two columns overlay perfectly. Those words will be at a different "depth"; the message literally pops out at you.

I could not do it as well. My monitor is too far away I guess (about 60-70 cm).