  Diverse processes rely on the viscous flow of polymer solutions through
porous media. In many cases, the macroscopic flow resistance abruptly increases
above a threshold flow rate in a porous medium---but not in bulk solution. The
reason why has been a puzzle for over half a century. Here, by directly
visualizing the flow in a transparent 3D porous medium, we demonstrate that
this anomalous increase is due to the onset of an elastic instability. We
establish that the energy dissipated by the unstable flow fluctuations, which
vary across pores, generates the anomalous increase in flow resistance through
the entire medium. Thus, by linking the pore-scale onset of unstable flow to
macroscopic transport, our work provides generally-applicable guidelines for
predicting and controlling polymer solution flows.
