Sea balls also called aegagropila, can be found on Mediterranean beaches. They are made of Posidonia fibers, which aggregate on the seabed due to the sea motions. To understand the mechanism of aggregation and compaction of these structures, we have performed a laboratory investigation on the dynamics of aggregation of fibers by surface gravity waves generated in a water tank. Amazingly, depending on the flexibility of the fibers, two different sites of aggregation are observed. Following our experiments, we propose an analytical derivation of the clustering of particles by the Stokes drift. This theoretical calculation is quite general and emphasizes the respective roles of the Stokes number and the density of the particles; it also underlines the importance of the fiber flexibility in their drift and explains our experimental observations.