Underground viscous hydrocarbon deposits, such as the viscous residues in conventional oil wells, are heated by mechanical wave energy to fluidize the hydrocarbons thereby to facilitate extraction thereof. For uniform, circular, symmetrical dispersion of mechanical wave energy of high-power and low-frequency, a mechanical wave energy radiator is provided comprising a cylindrical elastic tube of springy steel or the like preferably dimpled or corrugated and closed at one end and containing a liquid medium. Mechanical wave energy is applied to the liquid medium by a reciprocating source or the like connected to the radiator by a rigid walled tubular pipe or the like. The axial length of the radiator tube should be an odd multiple of one-quarter wavelength of the mechanical wave energy transmitted. Cavitation within the liquid is avoided by biasing the system with a steady state pressure at least as great as the maximum negative pressure swing of the mechanical waves in the liquid. Transformers are disclosed for accommodating changes in pipe diameter and changes in liquid medium throughout the system.