Bulk nanobubbles (BNBs) are submicron gaseous domains dispersed in solutions, which are supposed to survive for several hours or even days. In recent years, there has been a rapid growth in the research and extraordinary applications of BNBs. Conventional theories based on gas diffusion and Laplace pressure, however, predicted that nanoscale gas bubbles in water should dissolve within microseconds, presenting a modern-day paradox in current nanobubbles researches. Also, it is still challenging to efficiently produce BNBs and determine their gaseous nature with the available techniques. In this review, we start from a general introduction and brief history of nanobubbles researches and revisit the current progress on the generation methods and detection techniques. Two possible formation mechanisms are suggested, and the plausibility of the proposed theories on BNBs stability is discussed with some suggestions for future studies on bulk nanobubbles.