Bulk nanobubbles (BNB) are spherical bubbles of a sub-micrometer size, typically 100 nanometers, suspended in aqueous solution. Its discovery originated from the early studies on ultrasound and cavitation in 1962. Johnson and Cooke then reported indirect evidence for the existence of bulk tiny bubbles less than 1000 nm in seawater by navy sonar. After 2000, experimental reports on nanobubbles emerged due to the extensive use of advanced experimental techniques, such as dynamic light scattering (DLS), phase microscopy, infrared spectroscopy and nanoparticle tracking analyzers (NTA). Bulk nanobubbles have aroused strong interest due to their long lifetime and numerous applications, such as water treatment, agriculture, biomedicine, and micro-reactors. The unexpected stability of bulk nanobubbles seems to contradict the classical theory. According to the Epstein-Plesset theory, a bubble of 100 nm in radius will have an internal pressure about 14.4 times the atmospheric pressure, and it would dissolve within 1 μs. However, the existence period of observed nanobubbles ranges from several hours to weeks in numerous experiments. Such a long lifetime indicates that the reported bulk nanobubbles are in fact thermodynamically stable.