In a 24deg2 survey for baryonic matter at 33GHz in the Corona Borealis supercluster (CrB-SC) of galaxies (z=0.07), with the Very Small Array (VSA) interferometer (Génova-Santos et al. 2005, MNRAS 363, 79; 2008, arXiv: 0804.0199), we found a very strong temperature decrement in the Cosmic Microwave Background (CMB). It has an amplitude of-230±23μK and is located near the center of the supercluster, in a position with no known galaxy clusters, and without a significant X-ray emission in the ROSAT All-Sky Survey. Monte-Carlo simulations discard the primordial CMB Gaussian field as a possible explanation for this decrement at a level of 99.6%. We therefore concluded that this could be indicative of a Sunyaev-Zel'dovich (SZ) effect produced either by a warm/hot gas distribution in the intercluster medium or by a farther unknown galaxy cluster. Here we present an optical study of the galaxy distribution in this region, aiming at elucidating whether it traces a possible warm/hot gas filamentary distribution or a galaxy cluster. First, we have studied the galaxy population down to r≤20 magnitudes in the SDSS. This reveals an overdensity by a factor of 2 with respect to nearby control fields, but lower than in the galaxy clusters member of the CrB-SC. This indicates that the associated gas could at least be partially responsible for the observed CMB decrement. Second, we obtained spectroscopic redshifts, with the William Herschel Telescope (WHT), for a sample of galaxies in the region of the cold spot, and found evidence of a substructure with redshifts extending from 0.07 to 0.10. This suggests the existence of a dense filamentary structure with a length of several tens of Mpc. Finally, we investigated the presence of at least one farther cluster in the same line-of-sight, at z≈0.11.