My time has come!
The above stereographic image is for cross-eyed viewing (most stereograms are wall-eyed, so you may need to put your finger in front of your screen until this one comes into focus)
This is an image of Honolulu, Hawaii, published by NASA. Note Diamond Head (the volcanic crater) in the south.
Here are some other stereopairs published by JPL:
Wheeler Ridge, California
Mount Saint Helens
Salt Lake Valley, Utah
Wellington, New Zealand
Yupp, I never got the hang of cross-eyed viewing, even with the tips that are around, whereas the "looking through the image" technique is super easy for me, basically just relaxing my eyes. I assume there's people where it is the other way around, and the cross-eyed method works better for them.
Basically it's about which image is transferred as information from which of your eyes, and the two different techniques swap the eyes, which also swaps the 3D depth information.
I love the Wellington here viewed the "wrong" way - like the ocean is a massive plateau surrounding the coast, with that strip of developed area rising like another giant wall.