  We report on high-resolution spectroscopy of ultracold fermionic
\nak~Feshbach molecules, and identify a two-photon pathway to the rovibrational
singlet ground state via a resonantly mixed \Bcres intermediate state.
Photoassociation in a $^{23}$Na-$^{40}$K atomic mixture and one-photon
spectroscopy on \nak~Feshbach molecules reveal about 20 vibrational levels of
the electronically excited \ctrip state. Two of these levels are found to be
strongly perturbed by nearby \Bsing states via spin-orbit coupling, resulting
in additional lines of dominant singlet character in the perturbed complex
{${\rm B}^1\Pi |v{=}4\rangle {\sim} {\rm c}^3\Sigma^+ | v{=}25\rangle$}, or of
resonantly mixed character in {${\rm B}^1\Pi | v{=}12 \rangle {\sim}{\rm
c}^3\Sigma^+ | v{=}35 \rangle$}. The dominantly singlet level is used to locate
the absolute rovibrational singlet ground state ${\rm X}^1\Sigma^+ | v{=}0,
J{=}0 \rangle$ via Autler-Townes spectroscopy. We demonstrate coherent
two-photon coupling via dark state spectroscopy between the predominantly
triplet Feshbach molecular state and the singlet ground state. Its binding
energy is measured to be 5212.0447(1) \cm, a thousand-fold improvement in
accuracy compared to previous determinations. In their absolute singlet ground
state, \nak~molecules are chemically stable under binary collisions and possess
a large electric dipole moment of $2.72$ Debye. Our work thus paves the way
towards the creation of strongly dipolar Fermi gases of NaK molecules.
