Obviously, the classical theory cannot explain the thermodynamic stability of bulk nanobubbles, and new theories are needed. In the past 20 years, the proposed mechanisms for interpreting bulk nanobubble's stability are mainly divided into two categories: the enrichment of interfacial charges (zeta potential) and the adsorption of contaminants. Although these models are partly capable of interpreting why bulk nanobubbles are stable, there are some phenomena that cannot be explained by these models: the stability of bulk nanobubbles at low gas saturation or at zeta potential close to 0, and their almost constant size (~ 100 nm) from different reports. In detail, according to these models the stable radius of nanobubbles always depends on the number of charges and that of contaminants enriched on the nanobubble interface. If the number of charges and contaminants adsorbed can be adjusted in a wide range, stable nanobubbles in any size can be possible. This is inconsistent with experimental observation that the typical nanobubble size is often around 100 nm, regardless of the concentration of the amphiphilic organics or salt solution added. For both the zeta potential mechanism and the contaminant mechanism, they require the dissolved gas to have a certain level of gas supersaturation, which provides an additional pressure (Laplace pressure) to balance the surface tension of the bubble interface. This is not enough to explain experimental observations that the stable bulk nanobubbles can be observed at rather low gas saturation (𝑆~0 ), which is common condition in practice due to the exposure of nanobubble solutions to the ambient atmosphere. These disagreements demonstrate the existence of some important characteristics unidentified for the stabilization mechanism of real nanobubbles. Hence, the stabilization mechanism of bulk nanobubbles is still an open question.