Without viewer it's also possible to see the depth in the pictures, in any case in the small ones.
Many pictures on this site are given in two formats. You should be able to see the smaller with your naked eyes. Unfortunally you need two eyes.
If you wear glasses, keep them on your nose.
Look at the picture, relaxed.
At first you see the two pictures, next to eachother.
Take a look through the pictures. Stare in the distance. The image becomes out of focus and you will see three images next to eachother.
Try to concentrate yourself, still relaxed, on the picture in the middle.
That one will become sharp.
In that sharp picture you will see the depth.