We study a system of hard-core particles sliding downwards on a fluctuating one-dimensional surface which is characterized by a dynamical exponent z. In numerical simulations, an initially random particle density is found to coarsen and obey scaling with a growing length scale ∼ t 1/z . The structure factor deviates from the Porod law in some cases. The steady state is unusual in that the density-segregation order parameter shows strong fluctuations. The two-point correlation function has a scaling form with a cusp at small argument which we relate to a power law distribution of particle cluster sizes. Exact results on a related model of surface depths provides insight into the origin of this behaviour.PACS numbers: 05.70. Ln, 64.75.+g, 05.70.Jk How do density fluctuations evolve in a system of particles moving on a fluctuating surface? Can the combination of random vibrations and an external force such as gravity drive the system towards a state with large-scale clustering of particles? Such large-scale clustering driven by a fluctuating potential represents an especially interesting possibility for the behaviour of two coupled systems, one of which evolves autonomously but influences the dynamics of the other. Semi-autonomous systems are currently of interest in diverse contexts, for instance, advection of a passive scalar by a fluid [1], phase ordering in rough films [2], the motion of stuck and flowing grains in a sandpile [3], and the threshold of an instability in a sedimenting colloidal crystal [4].In this paper, we show that there is an unusual sort of phase ordering in a simple model of this sort, namely a system of particles sliding downwards under a gravitational field on a fluctuating one-dimensional surface. The surface evolves through its own dynamics, while the motion of particles is guided by local downward slopes; since random surface vibrations cause slope changes, they constitute a source of nonequilibrium noise for the particle system. The mechanism which promotes clustering is simple: fluctuations lead particles into potential minima or valleys, and once together the particles tend to stay together as illustrated in Fig. 1. The question is whether this tendency towards clustering persists up to macroscopic scales. We show below that in fact the particle density exhibits coarsening towards a phase-ordered state. This state has uncommonly large fluctuations which affect its properties in a qualitative way, and make it quite different from that in other driven, conserved, systems which exhibit coarsening [5].It is useful to state our principal results at the outset. (1) In an infinite system, an initially randomly distributed particle density exhibits coarsening with a characteristic growing length scale L(t) ∼ t 1/z where z is the dynamical exponent governing fluctuations of the surface. For some of the models we study, the scaled structure factor varies as |kL(t)| −(1+α) with α < 1, which represents a marked deviation from the Porod law (α = 1) for coarsening systems [6]. Further, a finite...