With the compressed amphiphile monolayer model, we then analyze the thermodynamic (both mechanical and chemical) equilibrium of bulk nanobubbles with their surroundings. The unexpected stability of bulk nanobubbles at rather low gas saturation can be well interpreted as a result of the formation of the compressed amphiphilic monolayer, which is in fact tensionless. Therefore, gas supersaturation is no longer a necessary condition for stability of bulk nanobubbles.