  Many key environmental, industrial, and energy processes rely on controlling
fluid transport within subsurface porous media. These media are typically
structurally heterogeneous, often with vertically-layered strata of distinct
permeabilities -- leading to uneven partitioning of flow across strata, which
can be undesirable. Here, using direct in situ visualization, we demonstrate
that polymer additives can homogenize this flow by inducing a purely-elastic
flow instability that generates random spatiotemporal fluctuations and excess
flow resistance in individual strata. In particular, we find that this
instability arises at smaller imposed flow rates in higher-permeability strata,
diverting flow towards lower-permeability strata and helping to homogenize the
flow. Guided by the experiments, we develop a parallel-resistor model that
quantitatively predicts the flow rate at which this homogenization is optimized
for a given stratified medium. Thus, our work provides a new approach to
homogenizing fluid and passive scalar transport in heterogeneous porous media.
