The "Monster of the Id" that is created by Dr. Morbius's subconscious can be quite intimidating, even though it is invisible for most of its appearances. First, mysterious footsteps are seen approaching the C-57D ship, walking up the stairs into the ship. The invisible monster sneaks into sleeping quarters and steals government documents. Later, it sneaks in again and kills a man violently. The third time it attacks, the crew members of the C-57D start shooting at it while it is being zapped by the electric fence they have set up. It briefly becomes visible, appearing like a large bear/boar creature, faintly outlined. Later, when it attacks Dr. Morbius's lab, it destroys (melts) an indestructible series of doors. Meanwhile, during both of these scenes, the combination of strange electronic music and sound effects adds to the frightening qualities of the beast's appearances.