A granular material is a system composed of many solid particles interacting mainly
through contact forces. Therefore, the dissipation of energy usually plays a dominant role
in the dynamics of these materials. For this reason, in experiments done so far, when an
object impacts on a granular bed it eventually dissipates all its energy and comes to rest.
In contrast, when a dense enough object is placed inside a fluid it keeps falling,
asymptotically approaching a terminal velocity.
Here we present experiments of a heavy object falling into a silo full of expanded
polystyrene spherical particles. The density of the granular medium is so low that it can not
bear the weight of an intruder whose mass is beyond a threshold value, even if the object
is very deep in the silo. We systematically vary the mass of an object impacting in such a
granular bed and we find a transition between the commonly observed behavior where the
object stops at a given depth, and a situation where the object keeps falling and reaches a
terminal velocity, just as in a fluid.