Sufficent neuron density but more importantly the density of connections between them, in certain locations of the brain responsible for carrying out unconscious actions. In order for a part of the brain to store unconscious memories, it must be a part of the brain repsonsible for handling unconscious actions while at the same time having the capacity to store memories, and the level of communicaiton neccesary with the rest of the brain to take in that information in the first place.
Even the idea of the "subconscious" as definied in popular psychology has been called into question. The leading theory to explain the idea of the "subconscious" is that certain centers of the brain work independently from each other and its up to the frontal lobe to arrange the information gathered in process in a way that makes logical sense. This would however mean that there is infact no such thing as a "subconscious" and instead the brain is made up of multiple autonomic structures that are dependent on each other, kind of like how our internal organs are self-contained structures but dependent on the rest of the body to function and survive. In the same way the separate parts of the brain are distinct structures that still require the assistance of the other pieces to process and store information in a reasonable way.
Even that though is a vast oversimplification of what they're finding. Neurology is still a fairly young field.
