  The unpolarized spin environment surrounding a central spin qubit is
typically considered as an incoherent source of dephasing, however, precise
characterization and control of the spin bath can yield a resource for storing
and sensing with quantum states. In this work, we use nitrogen-vacancy (NV)
centers in diamond to measure the coherence of optically-dark paramagnetic
nitrogen defects (P1 centers) and detect coherent interactions between the P1
centers and a local bath of $^{13}$C nuclear spins. The dipolar coupling
between the P1 centers and $^{13}$C nuclear spins is identified by signature
periodic collapses and revivals in the P1 spin coherence signal. We then
demonstrate, using a range of dynamical decoupling protocols, that the probing
NV centers and the P1 spins are coupled to independent ensembles of $^{13}$C
nuclear spins. Our work illustrates how the optically-dark P1 spins, despite
being unpolarized, can be used to extract information from their local
environment and offers new insight into the interactions within a many-body
system.
