Is every knot isotopic to the unknot?

In 1974, D. Rolfsen asked: Is every knot in S³ isotopic (=homotopic through embeddings) to a PL knot or, equivalently, to the unknot? In particular, is the Bing sling isotopic to a PL knot? We show that the Bing sling is not isotopic to any PL knot: (1) by an isotopy which extends to an isotopy of $2$-component links with $lk=1$; (2) through knots that are intersections of nested sequences of solid tori. There are also stronger versions of these results. In (1), the additional component may be allowed to self-intersect, and even to get replaced by a new one as long as it represents the same conjugacy class in $G/[G',G'']$, where G is the fundamental group of the complement to the original component. In (2), the "solid tori" can be replaced by "boundary-link-like handlebodies", where a handlebody V⊂ S³ of genus g is called boundary-link-like if π₁(S³-V̄) admits a homomorphism to the free group Fg such that the composition π₁(∂ V)→π₁(S³-V̄)→ Fg is surjective.