A reactor system comprising two or more subterranean reactors for high pressure treatment of materials. The reactors each comprise an inlet tube, a reaction region and an outlet tube and are deep enough for material in the inlet and outlet tubes to produce significant hydrostatic pressure. The reactors are adapted to exchange heat which may occur through a shared heat transferring wall or a heat exchanger including circulating liquid. The invention also extends to a method of simultaneously running two processes in the reactor system. The preferred processes are hydrothermal dewatering of coal, especially brown coal, and coal liquefaction wherein exothermic heat product of one process contributes to the heating requirements of the other process. The invention also extends to a method of hydrothermal dewatering of coal when conducted in a subterranean reactor in conditions of decreasing pressure.