Mesoscale eddies play an important role in phenomena such as general circulation, momentum budgets, ocean water mass distribution, and water and nutrient transport. Based on the vertical structure of the density and current core, mesoscale eddies are classified into two categories: surface- and subsurface-intensified. The utilization of remote sensing has provided considerable information on surface-intensified eddies. However, little is known about subsurface eddies due to the lack of in-situ observations, although they have been found occasionally in the global ocean. Currently, subsurface eddies are a trending topic within the scientific community. This study reviews the scientific background and research progress on subsurface eddies, with a focus on the northwestern Pacific Ocean. The vertical structure, evolutionary process, and possible formation mechanism of subsurface eddies are summarized based on observational and modelling results. Their influence on marine biogeochemistry, marine sound propagation, and possible important scientific issues are also discussed.