  We propose new searches for $n^\prime$, a dark baryon that can mix with the
Standard Model neutron. We show that IsoDAR, a proposal to place an intense
cyclotron near a large-volume neutrino detector deep underground, can look for
$n\to n^\prime \to n$ transitions with much lower backgrounds than surface
experiments. This opportune neutron-shining-through-a-wall search would be
possible without any modifications to the primary goals of the experiment and
would provide the strongest laboratory constraints on the $n$-$n^\prime$ mixing
for a wide range of mass splitting. We also consider dark neutrons as dark
matter and show that their nuclear absorption at deep-underground detectors
such as SNO and Borexino places some of the strongest limits in parameter
space. Finally, we describe other $n^\prime$ signatures, such as neutrons
shining through walls at spallation sources, reactors, and the disappearance of
ultracold neutrons.
