Deep, medium resolution optical spectrophotometry has been carried out for several dozens PNe, allowing plasma diagnostics and abundance determinations using weak nebular continuum emission and optical recombination lines (ORLs) emitted by helium and heavy element ions following captures of electrons. Comparison with values deduced using the traditional method based on collisionally excited lines (CELs) shows that the long debated disparities, whereby electron temperature derived from the hydrogen recombination spectrum Balmer discontinuity is systematically lower than that deduced from the collisionally excited [OIII] nebular to auroral forbidden line ratio and whereby heavy element abundances relative to hydrogen deduced from ORLs are systematically higher than the corresponding values determined from CELs, are ubiquitous.

Many of these nebulae have also been observed in the mid- and far-IR using the Infrared Space Observatory (ISO), giving access to fine-structure lines (FSLs), which have been crucial to discriminate various scenarios proposed to explain the aforementioned disparities, including temperature fluctuations, density inhomogeneities and abundance variations.

In this talk I will summarize observational results and show that both temperature fluctuations and density inhomogeneities fail to explain all the observations and that there is direct observational evidence pointing to the presence of a new component of ionized gas, cold and highly enriched in helium and heavy elements, embedded in the main body of the nebulae.