We present interferometric imaging at 33GHz, with the new superextended configuration of the Very Small Array (VSA), of a very deep decrement in the cosmic microwave background (CMB) temperature. This decrement is located in the direction of the Corona Borealis supercluster, at a position with no known galaxy clusters, and was discovered by a previous VSA survey. A total area of 3deg2 has now been imaged, with an angular resolution of 7arcmin and a flux sensitivity of 5mJybeam-1. These observations confirm the presence of this strong and resolved negative spot at -37 +/- 5mJybeam-1 (-229 +/- 32μK). This structure is also present in the Wilkinson Microwave Anisotropy Probe 5-year data. The temperature of the W-band (94GHz) data at the position of the decrement agrees within 0.3σn with that observed by the VSA at 33GHz and within 0.2σn with the Sunyaev-Zel'dovich (SZ) spectrum. Our analyses show that it is a non-Gaussian feature in the CMB at a level of 4.4σ, where sigma accounts for primordial CMB fluctuations, thermal noise and residual radio source contributions. The probability of finding such a deviation or larger in simulations including Gaussian CMB is only 0.63per cent. Therefore, an explanation other than primordial Gaussian CMB is required. We have considered the possibility of an SZ effect generated in a diffuse, extended warm/hot gas distribution. This hypothesis is especially relevant, as the presence of such structures, if confirmed, could provide the location for a significant fraction of the missing baryons in the Local Universe. However, from the absence of X-ray emission in this region we conclude that the whole decrement cannot be generated solely via the SZ effect in such structure. Therefore, the most plausible scenario is a combination between a negative CMB feature and a SZ effect, probably generated by a warm/hot gas distribution.