  We report the novel preparation of single crystals of tetragonal iron
sulfide, FeS, which exhibits a nearly ideal tetrahedral geometry with S--Fe--S
bond angles of 110.2(2) $^\circ$ and 108.1(2) $^\circ$. Grown via hydrothermal
de-intercalation of K${_x}$Fe${_{2-y}}$S${_2}$ crystals under basic and
reducing conditions, the silver, plate-like crystals of FeS remain stable up to
200 $^\circ$C under air and 250 $^\circ$C under inert conditions, even though
the mineral "mackinawite" (FeS) is known to be metastable. FeS single crystals
exhibit a superconducting state below $T_c=4$ K as determined by electrical
resistivity, magnetic susceptibility, and heat capacity measurements,
confirming the presence of a bulk superconducting state. Normal state
measurements yield an electronic specific heat of 5~mJ/mol-K$^2$, and
paramagnetic, metallic behavior with a low residual resistivity of
250~$\mu\Omega\cdot$cm. Magnetoresistance measurements performed as a function
of magnetic field angle tilted toward both transverse and longitudinal
orientations with respect to the applied current reveal remarkable
two-dimensional behavior. This is paralleled in the superconducting state,
which exhibits the largest known upper critical field $H_{c2}$ anisotropy of
all iron-based superconductors, with $H_{c2}^{||ab}(0) /
H_{c2}^{||c}(0)=$(2.75~T)/(0.275~T)=10. Comparisons to theoretical models for
2D and anisotropic-3D superconductors, however, suggest that FeS is the latter
case with a large effective mass anisotropy. We place FeS in context to other
closely related iron-based superconductors and discuss the role of structural
parameters such as anion height on superconductivity.
