Two-in two-out: emergent monopoles and Pauling entropy
Spin ice materials (Dy₂Ti₂O₇, Ho₂Ti₂O₇) have magnetic moments on a pyrochlore lattice obeying the "ice rules": two spins point in, two out of each tetrahedron — analogous to proton disorder in water ice. This gives a macroscopic ground-state degeneracy (Pauling entropy ≈ R/2·ln(3/2)). Violations create emergent magnetic monopoles connected by Dirac strings.