  We present interferometric imaging at 33 GHz, 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 (Genova-Santos et
al.). A total area of 3 sq.deg. has now been imaged, with an angular resolution
of 7 arcmin and a flux sensitivity of 5 mJy/beam.
  These observations confirm the presence of this strong and resolved negative
spot at -41+/-5 mJy/beam (-258+/-29 muK), with a signal to noise level of 8.
This structure is also present in the WMAP 5-year data. The temperature of the
W-band (94 GHz) data at the position of the decrement agrees within 1.2-sigma
with that observed by the VSA at 33 GHz, and within 0.2-sigma with the
Sunyaev-Zel'dovich (SZ) spectrum.
  Our analyses show that it is a non-Gaussian feature in the CMB at a level of
4.8-sigma. The probability of finding such a deviation or larger in CMB
Gaussian simulations is only 0.19 per cent. Therefore, an explanation other
than primordial 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.
