Adamantium, which makes his bones ultra-strong. Here's more from Wikipedia: 'Adamantium is a fictional metal alloy appearing in American comic books published by Marvel Comics. It is best known as the substance bonded to the character Wolverine's skeleton and claws. Adamantium was created by writer Roy Thomas and artists Barry Windsor-Smith and Syd Shores in Marvel Comics' Avengers #66 (July 1969), which presents the substance as part of the character Ultron's outer shell.[1] In the stories where it appears, the defining quality of adamantium is its practical indestructibility.'