I first learned about stereograms in 2001. Back then, the term was new to me and I had no idea that it was possible to see 3D shapes on a 2D screen with the naked eyes.
What are stereograms you ask?
A diagram or computer-generated image giving a three-dimensional representation of a solid object or surface
Wikipedia, as usual, has a nice article explaining stereograms.
After a long week of trying hard, researching and learning various techniques, I managed to see my first stereogram: