Air is injected locally at the base of an immersed granular bed. The gas, which is forced to flow gently through the material, creates several paths between the grains. We observe that the latter gas venting results in the emission of bubbles in a localized region at the free surface. Additional experiments, performed in two dimensions, permit a direct visualization of the paths, and a theoretical approach shows that the typical size of the region at the free surface can be accounted for by a diffusionlike process. The diffusion coefficient is expressed as a function of the system parameters.