  Viscoelastic fluids exhibit elastic instabilities in simple shear flow and
flow through curved streamlines. Surprisingly, we found in a porous medium such
fluids show strikingly different hydrodynamic instabilities depicted by very
large sideways excursions and presence of fast and slow moving lanes which have
not been reported before. Particle image velocimetry (PIV) measurements through
a pillared microchannel, provide experimental evidence of such instabilities at
very low Reynolds number (< 0.01). We observe a transition from a symmetric
laminar to an asymmetric flow, which finally transforms to a nonlinear
aperiodic flow with strong lateral movements. The instability is characterized
by a rapid increase in spatial and temporal fluctuations of velocity components
and pressure at a critical Deborah number (De). Our experiments reveal the
presence of a fascinating interplay between pore space and fluid rheology.
