Ice berg
As it is known by all, Ice bergs are floating ice that are seen on ocean of Artic and Antarctic region. The ice bergs are dangerous for sailors since only a small part of the ice berg remains visible above water and most of it is submerged, away from the sight of the sailor.
Sigmund Freud conceptualized human mind in the similar way. From the topographical view point, the human mind is composed of two parts, the conscious and the unconscious. The unconscious is again divided into two part, the preconscious and the unconscious proper. The conscious part of the mind is only available to a human being.
The analogy of ice berg is often used to describe this theory. The part of ice berg which the sailor is able to see from the deck is metaphorically similar to the conscious of human mind, the part under water represent unconscious.