AbstractRegelation, i.e., ice melts under compression and freezes again when the pressure is relieved, remains puzzling since its discovery in 1850’s by Faraday. Here we show that hydrogen bond (O:H-O) cooperativity and its extraordinary recoverability resolve this anomaly. The H-O bond and the O:H nonbond possesses each a specific heat η_x(T/Θ_Dx) whose Debye temperature Θ_Dx is proportional to its characteristic phonon frequency ω_x according to Einstein’s relationship. A superposition of the η_x(T/Θ_Dx) curves for the H-O bond (x = H, ω_H ~ 3200 cm⁻¹) and the O:H nonbond (x = L, ω_L ~ 200 cm⁻¹, Θ_DL = 198 K) yields two intersecting temperatures that define the liquid/quasisolid/solid phase boundaries. Compression shortens the O:H nonbond and stiffens its phonon but does the opposite to the H-O bond through O-O Coulomb repulsion, which closes up the intersection temperatures and hence depress the melting temperature of quasisolid ice. Reproduction of the T_m(P) profile clarifies that the H-O bond energy E_H determines the T_m with derivative of E_H = 3.97 eV for bulk water and ice. Oxygen atom always finds bonding partners to retain its sp³-orbital hybridization once the O:H breaks, which ensures O:H-O bond recoverability to its original state once the pressure is relieved.