Iso-8

"Iso-8 has a lot of potential as a power source." "Or a weapon!" "Which is why we need to protect it at all costs. We can debate the finer points later."

- Nick Fury and Captain America Iso-8 is a powerful element.

History
S.H.I.E.L.D. held Iso-8 in a number of facilities that they used to create powerful weapons. R.A.I.D., the Serpent Society and U.L.T.I.M.A.T.U.M. attacked one of these facilities and stole several missiles, but Captain America and his allies managed to defeat them and recover the missiles.