  We report the discovery of an extremely curved jet in the radio-loud quasar
PKS2136+141. Multi-frequency Very Long Baseline Array (VLBA) images show a
bending jet making a turn-around of 210 degrees in the plane of the sky, which
is, to our knowledge, the largest ever observed change in the position angle of
an astrophysical jet. Images taken at six different frequencies, from 2.3 to 43
GHz, reveal a spiral-like trajectory, which is likely a sign of an intrinsic
helical geometry. A space-VLBI image, taken with the HALCA satellite at 5 GHz
and having comparable resolution to our ground-based 15 GHz data, confirms that
the bend is a frequency-independent structure. VLBA monitoring data at 15 GHz,
covering eight years of observations, show knots in the jet clearly deviating
from ballistic motion, which suggests that the bending may be caused by a
growing helical Kelvin-Helmholtz normal mode. The jet appearance suggests a
helical wave at a frequency well below the "resonant" frequency of the jet,
which indicates that the wave is driven by a periodic perturbation at the base
of the jet. We fit the observed structure in the source with a helical twist,
and we find that a simple isothermal model with a constant wave speed and
wavelength gives a good fit. The measured apparent velocities indicate some
degree of acceleration along the jet, which together with an observed change in
the apparent half-opening angle of the jet allow us to estimate the changes in
the angle between the local jet direction and our line of sight. We suggest
that the jet in PKS2136+141 is distorted by a helical Kelvin-Helmholtz normal
mode externally driven into the jet (e.g. by precession), and that our line of
sight falls within the opening angle of the helix cone.
