seems to be the dominant population in these samples [@fiore2000; @barger2001a; @tozzi2001; @rosati2002; @stern2002]. First examples of the long-sought class of high-redshift, radio-quiet, high-luminosity, heavily obscured active galactic nuclei (type-2 QSO) have also been detected in deep Chandra fields [@norman2002; @stern2002] and in the XMM-Newton deep survey in the Lockman Hole field [@hasinger2002].

In this paper we report on our optical identification work in the Chandra Deep Field South, which, thanks to the efficiency of the VLT, has progressed to the faintest magnitudes among the deepest X-ray surveys.

THE CHANDRA DEEP FIELD SOUTH (CDFS)
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The Chandra X-ray Observatory has performed deep X-ray surveys in a number of fields with ever increasing exposure times [@mushotzky2000; @hornschemeier2000; @giacconi2001; @tozzi2001; @brandt2001a] and has completed a 1 Msec exposure in the Chandra Deep Field South, CDFS [@giacconi2002; @rosati2002] and a 2 Msec exposure in the Hubble Deep Field North, HDF-N [@brandt2002]. The Megasecond dataset of the CDFS is the result of the coaddition of 11 individual Chandra ACIS-I exposures with aimpoints only a few arcsec from each other. The nominal aim point of the CDFS is $\alpha=3:32:28.0$, $\delta=-27:48:30$ (J2000). This field was selected in a patch of the southern sky characterized by a 