A pair of workout trunks, once belonging to Angelo Siciliano, that developed mass-altering qualities.
The trunks possess the ability to manipulate the density of anyone who wears them: by increasing the density, bullets bounce off the users body; by decreasing it, the user is capable of "leaping tall buildings in a single bound" (the tag-line of the original 1938 version of Superman, before he could fly). The danger comes from the fact that eventually the wearer won't be able to stop the increase of their own density. Everything around them will slowly be pulled towards them as their growing density begins to generate a gravity field. If the user reaches infinite density, he'll implode and the amount of gravity created will flatten an area the size of Detroit (about 142.87 square miles or 370.03 square km).
The trunks were inadvertently purchased at a yard sale with a bundle of other clothes by Sheldon, who used the trunks to take on the superhero persona of the "Iron Shadow."
The artifact first came to the attention of the Agents after police found a mugger with a lamp post coiled around him and a drug dealer was found hanging from a flag post (both classic Spider-Man methods of bad guy delivery to the cops).
Myka wore the trunks in "Cangku Shisi" in order to neutralize Chuck Yeager's Favorite LP. The alternate Valda left the record playing on Oliver Sacks' Record Player and whenever Pete and Myka got near the record it created a sonic boom that sent them flying. Claudia gave Myka the trunks so she could increase her density to the point her body was too heavy to be moved by the sonic booms the record was creating. After the record was neutralized, however, Myka had to neutralize the trunks quickly as her body had become so heavy the ground was beginning to shatter underneath her.