The self-organization of particles into patterns is commonly observed when they are subjected to oscillations or vibrations, in both industrial and environmental settings. A well-known case is the formation of sand ripples under waves at the beach. We study a simplified system in our laboratory experiments: a shaking box filled with water and heavy spherical particles. Instead of forming ripples, the particles form chains or bands that are aligned perpendicular to the shaking direction. We look in detail into the interactions between the flow and particles, to eventually better understand the organization of complex and large-scale patterns.