We report the discovery of an extremely curved jet in the radio-loud quasar PKS 2136+141. Multifrequency VLBA images show a bending jet making a turnaround of 210° in the sky plane, to our knowledge the largest ever observed change in an astrophysical jet's P.A. 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 HALCA at 5 GHz and having comparable resolution to our ground-based 15 GHz data, confirms that the bend is a frequency-independent structure. Eight years of VLBA monitoring data at 15 GHz 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, indicating 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 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 jet's apparent half-opening angle, allows estimation of the changes in the angle between the local jet direction and our line of sight. We suggest that the jet in PKS 2136+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.
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