  We report the discovery of a six-month-long mid-infrared transient,
SDWFS-MT-1 (aka SN 2007va), in the Spitzer Deep, Wide-Field Survey of the NOAO
Deep Wide-Field Survey Bootes field. The transient, located in a z=0.19 low
luminosity (M_[4.5]~-18.6 mag, L/L_MilkyWay~0.01) metal-poor (12+log(O/H)~7.8)
irregular galaxy, peaked at a mid-infrared absolute magnitude of M_[4.5]~-24.2
in the 4.5 micron Spitzer/IRAC band and emitted a total energy of at least
10^51 ergs. The optical emission was likely fainter than the mid-infrared,
although our constraints on the optical emission are poor because the transient
peaked when the source was "behind" the Sun. The Spitzer data are consistent
with emission by a modified black body with a temperature of ~1350 K. We rule
out a number of scenarios for the origin of the transient such as a Galactic
star, AGN activity, GRB, tidal disruption of a star by a black hole and
gravitational lensing. The most plausible scenario is a supernova exploding
inside a massive, optically thick circumstellar medium, composed of multiple
shells of previously ejected material. If the proposed scenario is correct,
then a significant fraction (~10%) of the most luminous supernova may be
self-enshrouded by dust not only before but also after the supernova occurs.
The spectral energy distribution of the progenitor of such a supernova would be
a slightly cooler version of eta Carina, peaking at 20-30 microns.
