When boiling occurs in a pool or gas is injected into a stagnant water pool, the actual liquid level, i.e., the two-phase mixture level, becomes higher than the original liquid level, i.e., the collapsed water level, and depends on the gas held in the liquid pool. The two-phase mixture level is an important indicator of the core cooling of a nuclear reactor and of the long-term operation of the filtered containment venting system under accident conditions. To clarify the relationship between the two-phase mixture and collapsed water levels, we conducted an air-water two-phase flow experiment in which we used different channel geometries, i.e., circular pipes and rod bundles, to inject air at atmospheric pressure into stagnant water. Then, we used visual observation to obtain the swell and fluctuation amplitude of the two-phase mixture level. The results indicate that with increases in the superficial gas velocity, the flow channel geometry significantly affects the swell of the two-phase mixture level, and that the dominant bubble scale in the rod bundle was influenced by both the hydraulic diameter and the channel-box scale. We compared the height and fluctuation amplitude of the two-phase mixture level with existing and newly acquired experimental data and models.