Hopefully, you will already understand that the tunnel effect is not similar to digging a hole in a wall.

The tunnel effect could perhaps be better described as easily seeping through a wall that should be impassable.

Let's think about it in this way. Imagine Boy A playing with a ball within a space confined by a wall. (There was a scene like this in the film, The Great Escape.) While the boy can throw the ball at the wall, he cannot throw it through the wall. Another boy, Boy B, who is outside the wall and can't see what is going on, can however tell from the sound and the way the wall shakes that a ball is being thrown against the wall on the other side. Boy B, if he's a very imaginative youngster, should be able to piece the sound and vibrations together and imagine the ball coming over to his side and flying into his hands, even if it actually remains on Boy A's side. Of course, in the physical world as we experience it, there is now way the ball would pass through the wall.

The interesting thing about the tunnel effect, however, is that the ball does actually fly into Boy B's hands. It is not a figment of the imagination.

The tunnel effect can be put to practical use in, for example, creating switches that require only the tiniest force to work. Another application is scanning tunneling microscopes (STMs), which by measuring the tunnel effects that take place between a specimen and the tip of a very thin probe, can "read" the surface of the specimen and create an image of it.