  A large population of heavily obscured, Compton Thick AGNs is predicted by
models of galaxy formation, models of Cosmic X-ray Background and by the
``relic'' super-massive black-hole mass function measured from local bulges.
However, so far only a handful of Compton thick AGNs have been possibly
detected using even the deepest Chandra and XMM surveys. Compton-thick AGNs can
be recovered thanks to the reprocessing of the AGN UV emission in the infrared
by selecting sources with AGN luminosity's in the mid-infrared and faint
near-infrared and optical emission. To this purpose, we make use of deep HST,
VLT, Spitzer and Chandra data on the Chandra Deep Field South to constrain the
number of Compton thick AGN in this field. We show that sources with high
24$\mu$m to optical flux ratios and red colors form a distinct source
population, and that their infrared luminosity is dominated by AGN emission.
Analysis of the X-ray properties of these extreme sources shows that most of
them (80$\pm15%$) are indeed likely to be highly obscured, Compton thick AGNs.
The number of infrared selected, Compton thick AGNs with 5.8$\mu$m luminosity
higher than $10^{44.2}$ erg s$^{-1}$ turns out to be similar to that of X-ray
selected, unobscured and moderately obscured AGNs with 2-10 keV luminosity
higher than $10^{43}$ erg s$^{-1}$ in the redshift bin 1.2-2.6. This ``factor
of 2'' source population is exactly what it is needed to solve the
discrepancies between model predictions and X-ray AGN selection.
