  Current studies on Submillimeter Galaxies (SMGs) mostly focus on bright
sources with 850 micron flux greater than 2 mJy, and the results have shown
that they are likely high redshift mergers with z > 2 and could be a dominant
population on star formation in the early Universe. However, bright SMGs only
contributes 20-30% of the 850 micron extragalactic background light (EBL),
meaning the bulk of the cosmic star formation still hidden by dust and our
current understanding is biased. We have started a program to study an unbiased
sample of highly-amplified and intrinsically faint SCUBA detected SMGs in the
field of massive lensing clusters. Here we report the newly obtained SMA
observations at 850 micron on one of our sample source, A2390-5, behind the
massive lensing cluster A2390. We successfully detect the source with a flux of
3.95 mJy. Surprisingly, it does not have any counterpart in any other
wavelengths even though there are tentative candidates, which implies a very
dusty and high-z nature. With less than 1" positional accuracy and the adoption
of z = 5, we obtain the amplification factor of 12 using current lensing model,
which makes A2390-5 a faint SMG with a de-lensed flux of 0.33 mJy. Together
with our previous detection on another faint SMG, both of them have no
counterpart in other wavelengths and their properties are very different than
previously thought from the single-dish data. We emphasize the importance of
direct submillimeter high-resolution studies on faint SMGs, which could be the
dominant population of the high-z star formation.
