Tunneling occurs when a particle somehow escapes from a region by crossing a barrier that would require more energy than the particle has. This is impossible from the point of view of classical physics, but follows quite simply when we consider the wave nature of particles.

When a tiny particle like an electron is confined to a certain region by a barrier the electron's wave-function extends beyond the barrier. We interpret the square of the wave-function as being proportional to the probability that the electron can be found at a particular spot. So, if the wave extends to the far side of the barrier there is a non-zero chance that the electron will eventually be found there.

A practical example of tunneling is a scanning tunneling microscope (STM). This consists of a tiny tip that is scanned over a surface and which can be used to image individual atoms. The degree to which electrons tunnel from the surface to the tip depends critically on the distance between the surface and the tip. In an STM the tunneling current is maintained at a constant level, which means the tip is a constant distance from the surface at all times. As the tip is scanned over the surface the tip's up and down motions mirror what is happening on the surface, so a picture of the surface can be obtained with resolution at the level of individual atoms.