Promethium is a magical metal found only in Belasco's dimension known as Otherplace. In Otherplace, it takes the form of a giant animal heart, hidden in a cavern deep below its surface. Removal of the promethium to another dimension will cause Limbo to vanish. The heart of Limbo was once sought by Doctor Doom as an inexhaustible energy source for his nation of Latveria. It is currently in the possession of Darkoth.
- The DC Comics universe has a metal also called Promethium
- There is a real-life element called Promethium, but its properties are nothing like those of the fictional metal.