  A codimension-two spiral spin-liquid is a correlated paramagnetic state with
one-dimensional ground state degeneracy hosted within a three-dimensional
lattice. Here, via neutron scattering experiments and numerical simulations, we
establish the existence of a codimension-two spiral spin-liquid in the
effective honeycomb-lattice compound Cs$_3$Fe$_2$Cl$_9$ and demonstrate the
selective visibility of the spiral surface through phase tuning. In the
long-range ordered regime, competing spiral and spin density wave orders emerge
as a function of applied magnetic field, among which a possible
order-by-disorder transition is identified.
