  It is known that every exotic smooth structure on a simply connected closed
4-manifold is determined by a codimention zero compact contractible Stein
submanifold and an involution on its boundary. Such a pair is called a cork. In
this paper, we construct infinitely many knotted imbeddings of corks in
4-manifolds such that they induce infinitely many different exotic smooth
structures. We also show that we can imbed an arbitrary finite number of corks
disjointly into 4-manifolds, so that the corresponding involutions on the
boundary of the contractible 4-manifolds give mutually different exotic
structures. Furthermore, we construct similar examples for plugs.
