We present the first laboratory evidence of internal gravity wave tunnelling through weakly stratified fluid patches and we derive analytic theories for energy transmission by the waves in two distinct circumstances. In one, the computed transmission coefficient is directly analogous to the textbook calculation for quantum tunnelling of a free electron incident upon a potential barrier. In the other, we consider the partial reflection and transmission of internal waves through a mixed region bounded by discontinuities in the density profile. The results reveal a linear resonance between vertically propagating internal waves and interfacial waves that exist on either flank of the mixing region. The resonance permits perfect transmission of internal waves that would otherwise strongly reflect from the weakly stratified region. We discuss a specific application of our results to deep convective storm-generated internal waves that tunnel through the mesosphere to the ionosphere.