A seismic source configured to generate seismic waves underground. The source includes a tank configured to be buried underground, the tank having a cavity; an actuation mechanism provided inside the cavity, wherein the actuation mechanism is configured to have at least one movable part that moves back and forth to generate a seismic wave having a desired frequency; and a first fluid provided inside the cavity and around the actuation mechanism and configured to transform the back and forth movement of the at least one movable part into a varying pressure that directly acts on walls of the tank.