  Spiral spin liquids are an exotic class of correlated paramagnets with an
enigmatic magnetic ground state composed of a degenerate manifold of
fluctuating spin spirals. Experimental realisations of the spiral spin liquid
are scarce, mainly due to the prominence of structural distortions in candidate
materials that can trigger order-by-disorder transitions to more conventionally
ordered magnetic ground states. Expanding the pool of candidate materials that
may host a spiral spin liquid is therefore crucial to realising this novel
magnetic ground state and understanding its robustness against perturbations
that arise in real materials. Here, we show that the material LiYbO$_2$ is the
first experimental realisation of a spiral spin liquid predicted to emerge from
the $J_1$-$J_2$ Heisenberg model on an elongated diamond lattice. Through a
complementary combination of high-resolution and diffuse neutron magnetic
scattering studies on a polycrystalline sample, we demonstrate that LiYbO$_2$
fulfils the requirements for the experimental realisation of the spiral spin
liquid and reconstruct single-crystal diffuse neutron magnetic scattering maps
that reveal continuous spiral spin contours -- a characteristic experimental
hallmark of this exotic magnetic phase.
