What is "inertron"

Inertron is a fictional metallic chemical substance found in the DC Comics Legion of Super-Heroes universe. It is the hardest, densest substance in the DC universe, and is often used by the Legion of Super-Heroes in the 30th century. The first mention of inertron was in Adventure Comics #336.

Inertron is frequently described as indestructible and impenetrable; however, this only applies to regular humans, lesser superhumans and 30th century known technology, as, on some occasions, Superboy and Mon-El could break an inertron container by using all their strength. In Adventure Comics #370, though, a triple steel layer inertron-sealed prison was described as too tough for Superboy or Mon-El to break through. Matter Eater Lad has been shown to be able to eat it, however. Karate Kid has broken shackles made of inertron one time.