On the kagome lattice (corner-sharing triangles), antiferromagnetic Ising spins cannot all satisfy J > 0 bonds simultaneously. This geometric frustration produces a massive ground-state degeneracy — a spin liquid without long-range order even at T=0.
Kagome antiferromagnets are prototypical spin liquids. Herbertsmithite (ZnCu₃(OH)₆Cl₂) is a near-perfect kagome antiferromagnet studied experimentally. Ground-state entropy ≈ 0.502 k_B per spin (Pauling estimate for ice-like rules).