  We demonstrate particle clustering on macroscopic scales in a coupled
nonequilibrium system where two species of particles are advected by a
fluctuating landscape and modify the landscape in the process. The phase
diagram generated by varying the particle-landscape coupling, valid for all
particle density and in both one and two dimensions, shows novel nonequilibrium
phases. While particle species are completely phase separated, the landscape
develops macroscopically ordered regions coexisting with a disordered region,
resulting in coarsening and steady state dynamics on time scales which grow
algebraically with size, not seen earlier in systems with pure domains.
