A vortex is a flow phenomenon that is very commonly observed in nature. More than a
century, a vortex ring that forms during drop splashing has caught the attention of many
scientists due to its importance in understanding fluid mixing and mass transport processes.
However, the origin of the vortices and their dynamics remain unclear, mostly due to the lack
of appropriate visualization methods. Here, with ultrafast X-ray phase-contrast imaging, we
show that the formation of vortex rings originates from the energy transfer by capillary waves
generated at the moment of the drop impact. Interestingly, we find a row of vortex rings along
the drop wall, as demonstrated by a phase diagram established here, with different power-law
dependencies of the angular velocities on the Reynolds number. These results provide
important insight that allows understanding and modelling any type of vortex rings in nature,
beyond just vortex rings during drop splashing.