Embodiments generally relate to propulsion tube units and propulsion devicesfor progressing instruments along passages,and associated methods of use. For example, the instruments may include,tools, sensors, probes and/or monitoring equipmentfor medical use (such as endoscopy) or industrial use (such as mining). Insome embodiments, the propulsion device may comprise anelongate tube defining a channel configured to accommodate a liquid and apressure actuator in communication with the channel. Thepressure actuator may be configured to selectively adjust a pressure of theliquid in the channel to alternatingly: reduce the pressureto induce cavitation and form gas bubbles in the liquid； and increase thepressure to collapse some or all of the gas bubbles back intothe liquid, thereby accelerating at least part of the liquid towards the firstend of the tube and transferring momentum to the tube toprogress the tube along the passage.
