Abstract: This invention is essentially an improved means for contacting solids, liquids, and gases at elevated pressures and at desired temperatures, for providing inherently stable pressures, for conserving the heat of chemical reactions, and for minimizing the power requirement for pumping liquids continuously through a high pressure zone. The invention is particularly useful for processes utilizing physical, chemical, and/or thermal treatment, under elevated pressures, of continuously flowing streams of large volume which may contain suspended solids.The invention is based on the realization that deep boreholes, suitably lined, can be utilized as a particularly efficient apparatus for physical, chemical, and/or thermal processes.