  The steep spectrum of neutrinos measured by IceCube extending from >1 PeV
down to ~10 TeV has an energy flux now encroaching on the Fermi isotropic GeV
background. We examine several implications starting from source energetics
requirements for neutrino production. We show how the environment of
extragalactic nuclei can extinguish ~10-100 TeV gamma rays and convert their
energy to X-rays for plausible conditions of infrared luminosity and magnetic
field, so that the Fermi background is not overwhelmed by cascades. We address
a variety of scenarios, such as for acceleration by supermassive black holes
and hadronic scenarios, and observations that may help elucidate the neutrinos'
shadowy origins.
