  Magnetic reconnection is one of the most important processes in
astrophysical, space and laboratory plasmas. Identifying the structure around
the point at which the magnetic field lines break and subsequently reform,
known as the magnetic null point, is crucial to improving our understanding
reconnection. But owing to the inherently three-dimensional nature of this
process, magnetic nulls are only detectable through measurements obtained
simultaneously from at least four points in space. Using data collected by the
four spacecraft of the Cluster constellation as they traversed a diffusion
region in the Earth's magnetotail on 15 September, 2001, we report here the
first in situ evidence for the structure of an isolated magnetic null. The
results indicate that it has a positive-spiral structure whose spatial extent
is of the same order as the local ion inertial length scale, suggesting that
the Hall effect could play an important role in 3D reconnection dynamics.
