  A study is presented of HI line observations of the nearby spiral galaxy NGC
3521 observed with the VLA as part of The HI Nearby Galaxy Survey. Clearly
evident in the HI data cube is the presence of an anomalous HI component that
is both diffuse and slow-rotating. The data cube is dynamically decomposed into
regular and anomalous HI components. A mass of M_HI=1.5 x 10^9 Msun is
estimated for the anomalous HI - 20 per cent of the total HI mass. Standard HI
data products and rotation curves are produced for each dynamical component. In
terms of circular rotation speed, the anomalous HI is found to lag the regular
HI by ~25 - 125 km/s. Three-dimensional models are generated and used to
determine the possible location of the anomalous HI. The results strongly
suggest it to be distributed in a thick disc with a scale-height of a few kpc
(~3.5 kpc). It is concluded that the anomalous HI in NGC 3521 constitutes a
slow-rotating halo gas component, consistent with similar findings for other
nearby galaxies. A study of the radial distribution of the anomalous HI shows
it to be spatially coincident with the inner regions of the stellar disc where
the star formation rate is highest. It is most likely a galactic fountain that
has deposited gas from the disc of the galaxy into the halo.
