I propose new searches for a hidden/dark neutron that can mix with the Standard Model neutron. IsoDAR, a proposal to place an intense cyclotron near a large-volume neutrino detector deep underground, can look for a neutron regeneration signal with much lower backgrounds than surface experiments. This neutron-shining-through-a-wall search would be possible without any modifications to the experiment and would provide the strongest laboratory constraints on the neutron-dark neutron mixing for a wide range of mass splittings. Considering also dark neutrons as dark matter, I show that their nuclear absorption at deep-underground detectors such as SNO and Borexino places some of the strongest limits. Finally,
I describe other signatures, such as neutrons shining through walls at spallation sources, reactors, and the disappearance of ultracold neutrons.