Batmanium is a metal element with supernatural capabilities, discovered and named by Powers International in their quest to find undiscovered elements in the "island of stability"—the concept that, at some point, elements stop being too radioactive to remain stable.
Batmanium can be utilized in the form of a liquid or a solid at room temperature. It is extremely dense, as shown when Powers International fashioned a large batarang for its public reveal to weigh two tons. Whatever industrial applications for the metal thus far remain unknown.
It was later discovered that Batmanium was one of the five divine metals that was required to form the portal to the Dark Multiverse, along with Dionesium, Electrum, Nth Metal, and Promethium. When one person was exposed to all five metals, that person would be transported to the Dark Multiverse and switch places with anyone waiting on the other side. Batman was unknowingly mantled to be the portal and was exposed to all the metals over the course of his career, being exposed to Batmanium last by the Court of Owls.
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