—Supercontinents are a unique feature of the planet Earth. A brief review of supercontinents formed since the Archean shows that before the Eocambrian, supercontinents, notably Gondwana and Rodinia, amalgamated through high-temperature mobile belts, all of them containing ultrahigh-temperature granulite occurrences. During the final stage of the amalgamation, the lower continental crust was brought to magmatic temperature (from ~900 to more than 1000 °C) during a variable time span, from less than 10 Ma in the recent shortlived orogens to more than 150 Ma in the Eocambrian (Gondwana) or Neoproterozoic (Rodinia) long-lived orogens. Ultrahigh-temperature granulites worldwide contain the same types of fluid inclusions, namely, dense CO2 and highly saline aqueous brines. The fluid amount in the peak metamorphic conditions is indicated by the amount of preserved fluid inclusions (especially CO2) and by the secondary effects caused by the fluids when they left the lower crust, including regional feldspathization, albitization or scapolitization, and formation of megashear zones, either oxidized (quartz–carbonate) or reduced (graphite veins). While some fluids may be locally derived either from mineral reactions or from inherited sediment waters, carbon isotope signature and petrographical arguments suggest that most fluids, both CO2 and high-salinity brines, are derived from carbonatite melts resulting from partial melting of metasomatized mantle. Ultrahigh-temperature metamorphism is critical for supercontinent amalgamation, but the associated fluid causes instability and disruption shortly after amalgamation.