In quantum mechanics, tunnelling is the standard mechanism that a particle uses to escape (travel) from one energy level, through an area (or barrier) of higher energy into a lower energy area. This will occur even if some zone located in the pathway has yet higher energy, representing a barrier to the particle.
The basis for tunnelling is this: The Schroedinger equation predicts the probability that a particle can be found in a certain location, or that the particle will have a certain energy level. Over a given period of time, that probability is high enough that the particle can tunnel out of a trap, even though the particle had insufficient energy by the classical mechanics physics theory to overcome that trap.
In another, equivalent way of describing the tunnelling phenomenon, the wave/matter duality of a particle comes into play. The particle could be thought-of, instead, as an energy-wave packet, at resonance. The particle would be described something like a cloud of mist, but composed of energy. The energy would be continually resonating in some preferred directions {orbitals}, re-enforcing the zone-like or shell-like presence of the energy.
The energy-mist cloud could be said to occupy a preferred orbital, either circular or mostly in one direction. Like a cloud of mist, this energy would have no definite shape or solidity, but could be pulled, pushed, or tugged by nearby neighbors.
The probability that the particle must exist is always 100%. Physically, the particle-cloud is spread out over time. We could say at some moments, the matter-energy-cloud could reside far enough away from it's supposed location that it could tunnel away.
Effectively the particle behaves like the present location where it is supposed to exist is next-to a lower energy location. The particle tunnels right through whatever barrier existed as if the barrier was not present.
Note that the barrier in this case was also composed of real matter-energy. This barrier also has a nature so that at very small wavelengths, it too behaves like it is fuzzy or indefinite at times.
In electronic circuits, tunnel diodes do exist, and that they are useful in circuits where the tunnelling effect is exploited. In addition, modern transistor and I.C. chips use these same quantum barrier tunnelling effects to accomplish their modern electronic tricks.
The C-R theory has no problem accepting or using the above mentioned type of atomic-scale tunnelling. We do, however find that some modern cosmologies distort the use of tunnelling in such a way as to elicit from us the question "Are you sure that the Emperor has on his new clothes?"
By the C-R theory, tunnelling cannot provide an adequate explanation for most of the expected black hole (non C-R) effects found in the universe.
See Chapter 13 for the best description of why we feel that this tunnelling hypothesis will create more problems than it will solve. Briefly, we believe on a macroscopic level, any tunnelling of gravitons or other particles from the singularity will violate conservation of energy.