Iso-8 is a powerful element.
S.H.I.E.L.D. doctor Bernhold Renner studied Iso-8 to discover its capabilities in human cloning. 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 Project P.E.G.A.S.U.S. and stole several nuclear warheads, and Captain America and his allies set out to defeat them and recover the warheads.