  Disordered magnetic states known as spin liquids are of paramount importance
in both fundamental and applied science. A classical state of this kind was
predicted for the Ising antiferromagnetic triangular model more than half a
century ago, while additional non-commuting exchange terms were proposed to
induce its quantum version -- a quantum spin liquid. These predictions have not
been yet confirmed experimentally. Here we report a discovery of such a state
in the structurally perfect triangular-lattice antiferromagnet
NdTa$_7$O$_{19}$. Its magnetic ground state is characterized by spin-1/2
degrees of freedom with Ising-like interactions and gives rise to low-frequency
spin excitations persisting down to the lowest temperatures. Our study
demonstrates the key role of strong spin-orbit coupling in stabilizing spin
liquids that result from magnetic anisotropy and highlights the large family of
rare-earth (RE) heptatantalates RETa$_7$O$_{19}$ as a novel framework for
realization of these states, which represent a promising platform for quantum
applications.
