Abstract. This paper introduces a new approach to finding knots and links with hidden symmetries using "hidden extensions", a class of hidden symmetries defined here. We exhibit a family of tangle complements in the ball whose boundaries have symmetries with hidden extensions, then we further extend these to hidden symmetries of some hyperbolic link complements.A hidden symmetry of a manifold M is a homeomorphism of finite-degree covers of M that does not descend to an automorphism of M . By deep work of Margulis, hidden symmetries characterize the arithmetic manifolds among all locally symmetric ones: a locally symmetric manifold is arithmetic if and only if it has infinitely many "non-equivalent" hidden symmetries (see [13, Ch. 6]; cf. The partial answers that we know are all negative. Aside from the figure-eight, there are no knots with hidden symmetries with at most fifteen crossings [6] and no two-bridge knots with hidden symmetries [11]. Macasieb-Mattman showed that no hyperbolic (−2, 3, n) pretzel knot, n ∈ Z, has hidden symmetries [8]. Hoffman showed the dodecahedral knots are commensurable with no others [7].Here we offer some positive results with potential relevance to this question. Our first main result exhibits hidden symmetries with the following curious feature.
(S).We use a family {L n } of two-component links constructed in previous work [3]. For each n, L n is assembled from a tangle S in B 3 , n copies of a tangle T in S 2 × I, and the mirror image S of S. Figure 1 depicts L 2 , with light gray lines indicating the spheres that divide it into copies of S and T . For n ∈ N and m ≥ 0, we will also use a tangle T n ⊂ L m+n : the connected union of S with n copies of T . For instance, L 2 contains a copy of T 1 (which is pictured in Figure 2 below) and of T 2 .Upon numbering the endpoints of T n as indicated in Figure 2, order-two even permutations determine mutations: mapping classes of ∂(B 3 − T n ) induced by 180-degree rotations of the sphere obtained by filling the punctures. Theorem 1.8. For n ∈ N, the mutation of ∂(B 3 − T n ) determined by (1 3)(2 4) has a hidden extension over a cover of B 3 − T n and for any m ∈ N, taking T n ⊂ L m+n , a hidden extension over a cover of S 3 − L m+n .