Geysers, unique hydrothermal features that episodically erupt steam and water, have fascinated scientists and the public for generations. Recent discoveries of laterally offset subsurface reservoirs (“bubble traps”) in geyser systems have driven renewed interest in the subsurface processes that operate within coupled conduit-reservoir systems. We use a novel laboratory geyser equipped with a bubble trap to study the fluid mechanics within geyser systems, including the oscillations that occur on timescales ranging from milliseconds to eruption cycles and the dynamics within the conduit and bubble trap during eruptions. By analyzing in situ pressure-temperature timeseries, we find that the conduit-bubble trap system generates two types of oscillatory behavior with direct analogs in natural systems — one associated with mechanical oscillations of conduit fluids and one associated with acoustic modes propagating within the fluids. We develop a mathematical model for the fluid mechanical oscillations that includes the effect of compressible steam in the bubble trap and apply our results to two natural geysers – Old Faithful Geyser in Yellowstone and El Jefe Geyser in El Tatio, Chile – to examine how the resonant frequencies of the fluid mechanical and acoustic modes can be used to constrain conduit length scales and gas–liquid mass fractions in the conduit fluids. A key outcome is that Old Faithful’s bubble trap must be nearly full of liquid and contain only a small volume of compressed vapor in order to generate the 1 Hz fluid mechanical oscillations observed prior to eruptions. Our results emphasize the value of obtaining pressure/temperature timeseries data in geyser conduits. We show that during eruptions, flow within the laboratory geyser conduit is unsteady, occurs in the slug or churn flow regime, and gives rise to alternating discharge of liquid-dominated and vapor-dominated fluids during eruptions. We suggest that phase separation within geyser conduits may give rise to pulsing or surging seen in natural geyser jets, including that of Old Faithful.