  Pushing two fluids with different density one against the other causes the
development of the Rayleigh-Taylor instability at their interface, which
further evolves in a complex mixing layer. In porous media, this process is
influenced by the viscous resistance experienced while flowing through the
pores, which is described by the Darcy's law. Here, we investigate the mixing
properties of the Darcy-Rayleigh-Taylor system in the limit of large P\'eclet
number by means of direct numerical simulations in three and two dimensions. In
the mixing zone, the balance between gravity and viscous forces results in a
non-self-similar growth of elongated plumes, whose length increases linearly in
time while their width follows a diffusive growth. The mass-transfer Nusselt
number is found to increase linearly with the Darcy-Rayleigh number supporting
a universal scaling in porous convection at high Ra numbers. Finally, we find
that the mixing process displays important quantitative differences between two
and three dimensions.
