They were called 3D illusion images, or stereograms, and required a special technique of viewing them to arrive at the hidden images.
Books and big posters got printed showing them and challenging viewers to discover the hidden images.
I actually secured a book which essentially was a collection of works by different authors and waited breathlessly for the explosion.
Indeed, there was a "bust" or explosion, but instead this became an ancillary victim among the many, laid waste when tech stocks careened downward at the start of the new century.
But anyway, test your visual acuity and look for the hidden 3D images in these images taken from here:
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If unsuccessful, please drop a line and will be glad to show some techniques.
UPDATE:
I have about a 100 of these glossy images which because of copyrights cannot be reproduced here. They can be viewed personally.
