The BC8 phase of carbon was theoretically proposed nearly 40 years ago but lacking direct experimental structural confirmation, leaving its high-pressure stability and the predicted diamond-BC8-liquid triple point unresolved. Constraining carbon’s terapascal phase diagram is important for planetary interiors and inertial confinement fusion. Here, we report decaying shock compression experiments on single-crystal diamond with [100] orientation up to 1600 GPa and 14000 K, supported by large-scale molecular dynamics simulations using a machine-learning interatomic potential. Reflectivity increases markedly from 600 GPa and 6000 K, indicating formation of a metastable liquid-like intermediate (MLI). The measured Hugoniot curve closely follows the negative slope of the diamond melting curve between 700 and 830 GPa, is compatible with a diamond-BC8-liquid triple point at approximately 830 GPa and 7200 K, and is consistent with the BC8 melting curve from 830 to 1000 GPa. These measurements provide the high-accuracy thermodynamic constraints for testing melting curve models near the predicted tripe point and support the existence of the predicted BC8 phase of carbon. Simulations suggest that the MLI provides a transient pathway associated with the diamond-to-BC8 transition. These findings provide unprecedented experimental thermodynamic constraints on carbon's high-pressure phase diagram, offer new insights into ultrafast phase transitions in condensed matter at terapascal pressures, and suggest that carbon may contribute to magnetic-field generation within carbon-rich exoplanets. Direct structural confirmation of BC8 through in situ X-ray diffraction remains an open challenge.
