Abstract: A method of extracting a liquid, such as oil, from an underground reservoir, is characterized by introducing a pump operated by extraneous energy into a well bore at a level which is below the dynamic level of the liquid therein, and operating the lift device to lift the liquid to the surface of the well bore while conserving the residual reservoir energy, i.e. the energy remaining which corresponds to that normally used in lifting the liquid from the lift device level to the dynamic level. The pump is one in which the liquid pressure at its inlet applies equal and opposite forces to the pump drive member so that the latter is driven solely by the external drive and does not dissipate the liquid reservoir energy existing at the pump inlet level. In this manner, the residual reservoir energy at the pump inlet level is exploited to increase the quantity of the liquid transported to the well bore.