Thanks to its outstanding angular resolution, the Atacama Large Millimeter/submillimeter Array (ALMA) has recently unambiguously identified a population of optically dark galaxies with redshifts greater than z = 3, which play an important role in the cosmic star formation in massive galaxies. In this talk I will present our study on the properties of the six optically dark galaxies detected in the 69 arcmin2 GOODS-ALMA 1.1mm continuum survey. While none of them are listed in the deepest H-band based CANDELS catalog in the GOODS-South field down to H = 28.16 AB, we were able to de-blend two of them from their bright neighbor and measure an H-band flux for them. We present the spectroscopic scan follow-up of five of the six sources with ALMA band 4. All are detected in the 2 mm continuum with signal-to-noise ratios higher than eight. One emission line is detected in AGS4 (νobs =151.44GHz with a S/N=8.58) and AGS17 (νobs =154.78GHz with a S/N=10.23), which we interpret in both cases as being due to the CO(6-5) line at zAGS4 = 3.556 and zAGS17 = 3.467, respectively. These redshifts match both the probability distribution of the photometric redshifts derived from the UV to near-infrared spectral energy distributions (SEDs) and the far-infrared SEDs for typical dust temperatures of galaxies at these redshifts. We present evidence that nearly 70 % (4/6 of galaxies) of the optically dark galaxies belong to the same overdensity of galaxies at z ~ 3.5. The most massive one, AGS24 (M⋆ = 1011.32 M⊙ ), is the most massive galaxy without an active galactic nucleus (AGN) at z > 3 in the GOODS-ALMA field. It falls in the very center of the peak of the galaxy surface density, which suggests that the surrounding overdensity is a proto-cluster in the process of virialization and that AGS24 is the candidate progenitor of the future brightest cluster galaxy (BCG).