Tunneling, also known as the tunnel effect, is a quantum mechanical phenomenon by which a tiny particle can penetrate a barrier that it could not, by any classical or obvious means, pass. Though seemingly miraculous, the effect does have some intuitive characteristics. For instance, thin barriers allow more particles to tunnel than do thick ones, and low barriers permit more tunneling than do high ones.

Tunneling does not generally show itself in the macroscopic world. It only starts to become a factor for microscopic items. Atoms can tunnel, as can electrons, but things such as tennis balls and grapes, easily seen with the naked eye, will not. For microscopic particles, the barrier heights are described in terms of energy instead of distance, but for conceptual purposes there is little difference.

It is important to note that the effect can only be understood with the aid of quantum mechanics...