  Extreme UV (EUV) and X-ray loops in the solar corona connect regions of
enhanced magnetic activity, but they are not usually rooted in the dark umbrae
of sunspots because the strong magnetic field found there suppresses
convection. This means that the Poynting flux of magnetic energy into the upper
atmosphere is not significant within the umbra as long as there are no light
bridges or umbral dots. Here we report a rare observation of a coronal loop
rooted in the dark umbra of a sunspot without any traces of light bridges or
umbral dots. We used the slit-jaw images and spectroscopic data from IRIS and
concentrate on the line profiles of O IV and Si IV that show persistent strong
redshifted components in the loop rooted in the umbra. Using the ratios of O
IV, we can estimate the density and thus investigate the mass flux. The coronal
context and temperature diagnostics of these observations is provided through
the EUV channels of AIA. The coronal loop, embedded within cooler downflows,
hosts supersonic downflows. The speed of more than 100 km s$^{-1}$ is on the
same order of magnitude in the transition region lines of O IV and Si IV, and
is even seen at comparable speed in the chromospheric Mg II lines. At a
projected distance of within $1"$ of the footpoint, we see a shock transition
to smaller downflow speeds of about 15 km s$^{-1}$ being consistent with mass
conservation across a stationary isothermal shock. We see no direct evidence
for energy input into the loop because the loop is rooted in the dark uniform
part of the umbra with no light bridges or umbral dots near by. Thus one might
conclude that we are seeing a siphon flow driven from the footpoint at the
other end of the loop. However, for a final result data of similar quality at
the other footpoint are needed, but this is too far away to be covered by the
IRIS field of view.
