  We report on the first stereoscopic observations of polar coronal jets made
by the EUVI/SECCHI imagers on board the twin STEREO spacecraft. The
significantly separated viewpoints ($\sim$ 11$^\circ$) allowed us to infer the
3D dynamics and morphology of a well-defined EUV coronal jet for the first
time. Triangulations of the jet's location in simultaneous image pairs led to
the true 3D position and thereby its kinematics. Initially the jet ascends
slowly at $\approx$10-20 $\mathrm{{km} {s}^{-1}}$ and then, after an apparent
'jump' takes place, it accelerates impulsively to velocities exceeding 300
$\mathrm{{km} {s}^{-1}}$ with accelerations exceeding the solar gravity.
Helical structure is the most important geometrical feature of the jet which
shows evidence of untwisting. The jet structure appears strikingly different
from each of the two STEREO viewpoints: face-on in the one viewpoint and
edge-on in the other. This provides conclusive evidence that the observed
helical structure is real and is not resulting from possible projection effects
of single viewpoint observations. The clear demonstration of twisted structure
in polar jets compares favorably with synthetic images from a recent MHD
simulation of jets invoking magnetic untwisting as their driving mechanism.
Therefore, the latter can be considered as a viable mechanism for the
initiation of polar jets.
